I have a clientlib in etc/designs/[site name]. In the clientlib I have several .less files (from the Twitter Bootstrap project) in a "css" directory. If I put "bootstrap.less" (wich has "@import" calls to the other .less files) into the css.txt CQ throws an "javax.jcr.RepositoryException: Error during assembly of /etc/designs/..." error. If I put the other .less files into the css.txt instead it stops half-way through the 2nd of the 34 .less files and none of the variable from "variables.less" (first file loaded) get passed to what little of the other files that do get loaded.
Adobe/Day have absolutely no documentation on how to use LESS in CQ and, other than a blog that mentions the @import bug, there is nothing on the web either.
Nevermind. Turns out that Twitter Bootstrap requires LESS.js 1.3.3. CQ5.5 only has 1.1.6 and, because it is baked into the JAVA HTML rendering backend, it is not upgradeable.