[Cel Skeggs]

Our team has had success with using RetroTranslator to let us write our core libraries in Java 5 and downgrade them to run on the cRIO - and it did, actually, work pretty reliably for us last year.
It has a few issues with anonymous classes, so we don't use in for our main robot coding, but it proved useful to write the CCRE's primary abstractions.
We also rewrote a large part of the collections library to run, though probably you could drop in the recent JRE implementations of this library if you aren't worrying about licensing issues.

Additionally, supposedly the eclipse compiler can target an earlier bytecode version than source version, which might be a more reliable method of doing this kind of retargetting.