I assume by VM you mean ZeroVM. Well, ZeroVM currently doesn't allow self modifying code at all. Nice try.... We haven't touched Google's provided validator in order not to break anything security-related. And if you think you have a good idea for vulnerability then you can claim some Google prizes.

If you meant more practical uses for it then unfortunately modern JIT would be difficult to support efficiently as they constantly recompile and with ZeroVM it is not only recompilation but also validation. However, JIT that recompile only once, on loading, is easy to support. In fact, next version of NaCl dumps GNU toolchain in favor for JITy LLVM, but then recompilation is happening only once.