For clarity, you don't really need a "browser-grade sandbox" for this -- the wasm2c route that Firefox/RLBox are using is quite lightweight. The only runtime support necessary is basically malloc/free + a signal handler to recover from OOB accesses (and even the signal handler is optional if you can tolerate a runtime slowdown). It's usually pretty easy to compile and link wasm2c's output with an executable that can speak the C ABI.

(I've contributed to wasm2c and think it's a cool tool.)