Assetic filter which incorporates Bless (http://blesscss.com/). Designed for use with Symfony and the AsseticBundle, but should be simple to adapt for other situations.

BlessFilter.php

<?php

namespace YourProj\YourBundle\Assetic\Filter;

use Assetic\Asset\AssetInterface;
use Assetic\Exception\FilterException;
use Assetic\Filter\FilterInterface;

/**
 * Bless splits up CSS files to get around the limit of 4096 selectors per file in IE<=9.
 * http://blesscss.com/.
 */
class BlessFilter implements FilterInterface
{
    const BLESS_OUTPUT_SUBDIR_NAME = 'bless-output';

    protected $pathForSplitFiles;

    /**
     * @param string $pathToWebDir The full path to the app's web directory
     */
    public function __construct($pathToWebDir)
    {
        // ensure that the path ends in a slash
        $pathToWebDir = rtrim($pathToWebDir, DIRECTORY_SEPARATOR) . DIRECTORY_SEPARATOR;

        $this->pathForSplitFiles = sprintf(
            '%s%s%s',
            $pathToWebDir,
            self::BLESS_OUTPUT_SUBDIR_NAME,
            DIRECTORY_SEPARATOR
        );
    }

    public function filterLoad(AssetInterface $asset)
    {
    }

    /**
     * {@inheritdoc}
     */
    public function filterDump(AssetInterface $asset)
    {
        // Set up files and directories
        // =============================

        $tempInputFile = tempnam(sys_get_temp_dir(), 'input');
        // File must have an extension of css for bless to accept it
        $inputFile = $tempInputFile . '.css';
        rename($tempInputFile, $inputFile);

        // We can't simply pass $asset->getSourcePath() to bless - that contains the original
        // less file.  We need to get the CSS from $asset->getContent().
        //
        // We write this CSS to a file - it's too much text to simply pipe to bless on the command
        // line.
        file_put_contents($inputFile, $asset->getContent());

        // ensure that the output directory exists
        if (!is_dir($this->pathForSplitFiles)) {
            mkdir($this->pathForSplitFiles, 0777, true);
        }

        // The output file isn't actually referenced by the resulting webpage; it's just a place
        // to dump the output from bless.  Any additional CSS files created will be placed
        // in the same directory.
        $outputFile = $this->pathForSplitFiles . basename($asset->getTargetPath());

        // Build and run the bless command
        // ================================

        // The resulting asset will reference any "split out" CSS files with @import statements
        // In dev, the additional CSS files will be real files but the initial asset (containing
        // the imports) will be served by the special Assetic controller and so won't actually
        // be a real file on disk.  In other environments, the initial asset is also dumped to a
        // real file by Assetic.

        $command = sprintf(
            "blessc %s %s",
            $inputFile,
            $outputFile
        );

        $output = [];
        $error = 0;
        $output = exec($command, $output, $error);

        if ($error) {
            throw new FilterException(sprintf(
                "Error when running blessc: %s",
                $output
            ));
        }

        // Set the content of the resulting asset, and tidy up
        // ====================================================

        $initialCSS = file_get_contents($outputFile);
        $initialCSS = $this->makeImportPathsAbsolute($initialCSS);
        $asset->setContent($initialCSS);

        // to avoid confusing people, remove $outputFile as it's not actually used
        unlink($outputFile);

        // we don't need $inputFile any more either
        unlink($inputFile);
    }

    /**
     * Bless creates an output file which references the other CSS files via import statements.
     * However, these statements use relative paths - in fact, they just refer to the other files
     * by name, and don't include any path information.  This means that it is expected that the
     * "chunked" files will be in the same folder as the main one.
     *
     * In dev, you can put the chunked files into web/css and the paths will work - Assetic will
     * serve the main file from the virtual directory web/css (in fact, it's served by a special
     * Assetic controller, it's not a file on disk).
     *
     * In test, however, the first CSS file attempts to import the chunked files from
     * app_test.php/css/, which doesn't map to the web/css folder and so the files aren't imported.
     *
     * To avoid this problem, alter the import statements to use absolute paths.  This also allows
     * us to store the chunked files in a different folder (self::BLESS_OUTPUT_SUBDIR_NAME).
     * This can avoid confusion since web/css then isn't used for both real files and "fake" files
     * intercepted by the Assetic controller.
     *
     * @param string $initialCSS
     *
     * @return string The CSS file with any bless-imports updated to be absolute references to files
     *                in self::BLESS_OUTPUT_SUBDIR_NAME.
     */
    private function makeImportPathsAbsolute($initialCSS)
    {
        $lines = explode(PHP_EOL, $initialCSS);
        $madeAChange = false;
        for ($i = 0; $i < count($lines); $i++) {
            $line = $lines[$i];
            $replaced = 0;
            $updatedLine = preg_replace(
                '/@import url\(\'([^\']*blessed\d[^\']*)\'\);/',
                '@import url(\'/' . self::BLESS_OUTPUT_SUBDIR_NAME . '/$1\');',
                $line,
                -1,
                $replaced
            );
            if ($replaced) {
                $lines[$i] = $updatedLine;
                $madeAChange = true;
            } else {
                break; // stop parsing the lines as soon as we reach one which isn't a bless import
            }
        }

        if ($madeAChange) {
            return implode(PHP_EOL, $lines);
        } else {
            return $initialCSS;
        }
    }
}

config.yml

assetic:
    debug:          %kernel.debug%
    use_controller: false
    bundles:        [ YourBundle ]
    filters:
        cssrewrite: ~
        less:
            node:       %node_bin%
            node_paths: [%node_path%]
            apply_to:   '\.less$'
        # The order here matters - we want to parse the less file after the less filter has been
        # applied to it.
        #
        # Note that, with this order, the bless filter will only be called if the less file hasn't
        # already been converted to CSS.  Therefore, to ensure the filter runs afresh (e.g. for
        # debugging it or if you make changes), you'll need to clear the Symfony cache and your
        # browser cache.
        bless:
            resource: 'assetic.xml' # workaround for assetic to accept our filter - see https://github.com/symfony/AsseticBundle/issues/50
            apply_to:  '\.less$'

services.yml

    assetic.filter.bless:
        class: YourProj\YourBundle\Assetic\Filter\BlessFilter
        arguments: [%kernel.root_dir%/../web]
        tags:
            - { name: assetic.filter, alias: bless }

It looks like the command format has changed in Bless 4, so the code above will need adapting if you're on that version. (The above was written against 3.0.3.)