We believe it should be possible in many JVM languages, but I don't think anyone at VoltDB has tried personally. It's certainly something we'd like to try out and more fully support down the road, but I don't have a timeframe.
There is a post with an example stored procedure in Scala here: http://www.ruimo.com/howto/voltdb/index_en.html
If you want to try to write a procedure in another JVM language, we'll try to be as supportive as possible. We're very interested in the results. Let us know if you have questions.