MiddleIncludeProcessor.rb

require 'sprockets/directive_processor'

class MiddleIncludeProcessor < Sprockets::DirectiveProcessor

  def prepare
    @pathname = Pathname.new(file)
    @header = ''
    @body = data
    @included_pathnames = []
    @compat             = false
  end

  def evaluate(context, locals, &block)
    @context = context
    @lines ||= body.lines.each_with_index.map do |line, index|
      if directive = line[DIRECTIVE_PATTERN, 1]
        name, *args = Shellwords.shellwords(directive)
        if name == 'include'
          context.evaluate(args.first)
        end        
      else 
        line
      end
    end
    @result = @lines.compact.join
  end

end

sprockets = Sprockets::Environment.new
sprockets.register_processor('application/javascript', MiddleIncludeProcessor)