There is IcedTea available in the repos (this includes the java command to run a java applications). I use this to run my (hobby) java apps.

I've tried Alchemy, and it runs out-of-the-box on the N900, however the menu is not accessible.

Maybe an Alchemy developer (or yourself) can add another way to access the menu that works for the N900...

What needs to be done after that (I think) is packaging and getting it in the Extras-Devel repository so everyone can benefit.