I don't really get the problem though. Java is trivial to learn for anyone. It's not your main product, it's not going to take a massive amount of time for a single plugin. Just write it and be done...
It might be really easy to learn. But after having programmed Ruby for any significant time Java is very unpleasant to program ;)
Also just learning basics to do simple customizations is not enough. If something breaks you will not have a slightest idea how to debug and fix. With a Ruby CI server I can just stick a debugger and walk through the code.
I don't really get the problem though. Java is trivial to learn for anyone. It's not your main product, it's not going to take a massive amount of time for a single plugin. Just write it and be done...