This may be related to "Failure with Sass @import #6". It looks like the same is happening w/ LESS. @imports are failing unless executing from a base directory.
My current converted (which is working):
class LessConverter < JekyllAssetPipeline::Converter
Dir.chdir "../_assets/stylesheets" do
return Less::Parser.new(:paths => ['.', '_assets']).parse(@content).to_css
May be worth adding a LESS sample to the README as well.