LESStmbundle.tmbundle.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>beforeRunningCommand</key>
	<string>saveActiveFile</string>
	<key>command</key>
	<string># This uses the cli for the less compiler 
# it should work with either the node module or the ruby gem (which is really old)

# Fucking Textmate doesn't seem to like TM_PATH
less="${TM_PATH:-$TM_SELECTED_FILE}"
css="${less%.less}.css"

# Here's where the magic happens
lessc "$less" &gt; "$css"
echo "lessc $less &gt; $css"
echo "Done"</string>
	<key>input</key>
	<string>document</string>
	<key>keyEquivalent</key>
	<string>@s</string>
	<key>name</key>
	<string>Save to CSS (sh)</string>
	<key>output</key>
	<string>showAsTooltip</string>
	<key>scope</key>
	<string>source.css.less</string>
	<key>uuid</key>
	<string>9F9EEC0F-8976-41A0-A541-B5FA04A04E17</string>
</dict>
</plist>

If anybody stumbles onto this and wonders why I wrote it (rather than just use the existing bundle, which I've actually contributed to), it's because Textmate wasn't finding the less node module.
Since I'm stupid, I did this rather than solve the real problem.