That UXN thing looks like it could be trivially jitted on-the-fly. Unless I am misreading their site, and the code doesn't have to reside in the ROM, of course.
It's a Von Neumann machine with no instruction cache coherence instructions. JITs for these are not trivial to produce due to every memory store potentially invalidating the JIT code, so you need clever solutions to make that invalidation extremely fast.