wasm has no way to remap writable memory as executable, but you can absolutely call back into javascript to instantiate and link a new executable module, like https://github.com/remko/waforth does.
Yes, I understand that you can do anything with imports. But that's not part of the Wasm spec. That's a capability the host has decided to give the module. Of course the person with the most privilege can always open holes up, but that capability is not there by default.
