[nezzor]

It does exist: https://github.com/llvm-dcpu16/llvm-dcpu16. The only problem is that there's not going to exist a compiler in-game. People will have to import the compiled assembly code, which isn't nearly as fun as staying in the game during the whole process.

[RodgerTheGreat]

Which is a pretty good argument for building a self-hosted Forth. You get a compiler and REPL suitable for commanding the computer in a package that will fit easily in the limited memory resources available. If you use threaded code with an "inner interpreter", it's possible to generate very compact code and fit a lot in a small space, or you could emit machine code directly ("subroutine threaded code") for speed.

[Scaevolus]

Someone's done that! http://matt.immute.net/files/goforth/