Is the JRuby environment sufficiently different that you can't TDD against MRI and then save JRuby for final validation testing and deployment? They shouldn't be that different, unless you're not writing pure ruby code.

Alternatively, Nailgun should reduce a lot of that pain. Effectively you have a running server instance of JVM and run some clients on it. That gives you JVM speed advantages without the slow startup. JRuby natively supports it: https://github.com/jruby/jruby/wiki/JRubyWithNailgun