GearUp is a simple Gem that will start a thin server running sprockets (rails asset-pipeline).
What does this means for you?
You have "some-directory" with some assets:
Switch to that directory:
$ cd <some-directory>
Gem install 'gearup':
$ gem install gearup
Played with on JRuby 188.8.131.52, 1.9.2, 1.9.3.
Let me know, if your using it and/or if you have any trouble.