Less with assets pipeline

application.css

/*
 *= require bootstrap
 */

bootstrap.css.less

/*
 * app/assets/bootstrap.css.less
 */

@import "bootstrap/bootstrap.less";

less.rb

# config/initializers/less.rb

require 'tilt/css'

module Tilt
  class LessTemplate < Template
    def prepare
      if ::Less.const_defined? :Engine
        @engine = ::Less::Engine.new(data)
      else
        # this line was the key :-)
        parser  = ::Less::Parser.new(:filename => eval_file, :line => line, :paths => [Rails.root.join('app','less')])
        @engine = parser.parse(data)
      end
    end  
  end  
end

notes.txt

Place the bootstrap/lib/*.less files in app/less/bootstrap/*.less