Another experiment with Gulp.js, but this one leaves control to the developer. I don't have exec() right yet (not synchronous!) Seems to be in a more recent NodeJS that I am running.

Gulpfile-config.js

// Usage:
//   grunt deploy - invokes magento dev:source-theme:deploy for all themes.
//   grunt less   - runs less over all themes.
//   grunt watch  - watches for file changes with livereload.


// =========================== Configuration ===========================

// The default Less processing pipeline.
function lessPipeline(srcLessFile, destDir) {
    return function() {
        return gulp.src(srcLessFile)
                   .pipe(less())
                   .pipe(sourcemaps.write())
                   .pipe(livereload())
                   .pipe(gulp.dest(destDir));
    };
}

// Areas, themes, locales, CSS to genernate, and files to watch.
var config = {
    'frontend': { // Area
        'Magento/blank': { // Theme
            'en_US': { // Locale comma list
            }
        },
        'Magento/luma': { // Theme
            'en_US,en_AU': { // Locale comma list
                //'css/kangaroo': { // Target CSS file
                    //'watch': ['**/*.less'],
                    //'pipeline': lessPipeline
                //}
            }
        }
    }
};

// =========================== End ===========================



// Note: You should need to change the following - it is driven from
// configuration above.

// Include gulp
var gulp = require('gulp');

// Include plugins
var jshint = require('gulp-jshint');
var less = require('gulp-less');
var concat = require('gulp-concat');
var uglify = require('gulp-uglify');
var rename = require('gulp-rename');
var gutil = require('gulp-util');
var livereload = require('gulp-livereload');
var sourcemaps = require('gulp-sourcemaps');

// Include child process invocation support
var exec = require('child_process').exec;
var sys = require('sys');
var util = require('util');


// Helper function for reporting output of child processes.
function logger(error, stdout, stderr) {
    sys.print(stdout);
    sys.print(stderr);
    if (error !== null) {
        console.log('exec error: ' + error);
    }
}

function shell(cmd) {
    console.log(cmd + "\n");
    var log = exec(cmd, logger);
}

// Fill in the standard Magento files. You can override these defaults
// by listing them with 'null' as the value.
for (var area in config) {
    for (var theme in config[area]) {

        // Add in missing standard Magento files.
        for (var locale in config[area][theme]) {
            var obj = config[area][theme][locale];
            // Add rules for stanard Magento files.
            var std = ['css/styles-l', 'css/styles-m', 'css/email', 'css/print'];
            for (var i = 0; i < std.length; i++) {
                css = std[i];
                if (obj.hasOwnProperty(css)) {
                    // 'null' in config means we really don't want it.
                    if (obj[css] == null) {
                        delete obj[css];
                    }
                } else {
                    // Add in default rule for Magento files.
                    obj[css] = {
                        'watch': ['**/_*.less', css + '.less'],
                        'pipeline': lessPipeline
                    };
                }
            }
        }

        // Now expand any comma list of locale names.
        for (var localeList in config[area][theme]) {
            var obj = config[area][theme][localeList];
            delete config[area][theme][localeList];
            var locales = localeList.split(',');
            for (var i = 0; i < locales.length; i++) {
                config[area][theme][locales[i]] = obj;
            }
        }
    }
}


// Create rules per theme/locale/isCustom
// (less-<area>-<theme>-<locale>-<css>).
var lessTargets = [];
for (var area in config) {
    for (var theme in config[area]) {
        for (var locale in config[area][theme]) {
            for (var css in config[area][theme][locale]) {
                var target = 'less-' + area + '-' + theme + '-' + locale + '-' + css;
                lessTargets.push(target);

                var pipeline = config[area][theme][locale][css]['pipeline'];

                var baseDir = 'pub/static/' + area + '/' + theme + '/' + locale;

                var srcFile = baseDir + '/' + css + '.less';

                var cssDir = '';
                if (css.lastIndexOf('/') >= 0) {
                    cssDir = css.substr(0, css.lastIndexOf('/'));
                }
                var destDir = baseDir + cssDir;

                gulp.task(target, pipeline(srcFile, destDir));
            }
        }
    }
}


// Make 'less' target depends on all theme/locale variations.
gulp.task('less', lessTargets);


// Watch every watch pattern and trigger recompilation as required.
gulp.task('watch', function() {
    livereload.listen();
    for (var area in config) {
        for (var theme in config[area]) {
            for (var locale in config[area][theme]) {
                for (var css in config[area][theme][locale]) {
                    var watches = config[area][theme][locale][css]['watch'];
                    var baseDir = 'pub/static/' + area + '/' + theme + '/' + locale;
                    var target = 'less-' + area + '-' + theme + '-' + locale + '-' + css;

                    for (var i = 0; i < watches.length; i++) {
                        var watch = watches[i];
                        gulp.watch(baseDir + '/' + watch, [target]);
                        console.log('WATCH:  ' + baseDir + '/' + watch + '\n');
                        console.log('TARGET: ' + target + '\n');
                    }
                }
            }
        }
    }
});


// Run Magento deploy command on every theme and language code.
gulp.task('deploy', function() {
    for (var area in config) {
        for (var theme in config[area]) {

            var rmCmd = 'rm -rf pub/static/' + area + '/' + theme;
            shell(rmCmd);

            for (var locale in config[area][theme]) {
                var files = Object.keys(config[area][theme][locale]);

                var deployCmd = 'magento dev:source-theme:deploy'
                    + ' --type=less'
                    + ' --area=' + area
                    + ' --theme=' + theme
                    + ' --locale=' + locale
                    + ' ' + files.join(' ');

                shell(deployCmd);
            }
        }
    }
});


// Default Task
gulp.task('default', ['less', 'watch']);