Just in case anyone is interested, it is now possible to build Rebol for WebAssembly.
Demo is available here: https://rebol.tech/wasm/
Source is for now in a separate branch: https://github.com/Oldes/Rebol3/tree/wasm
It has its own `nest` file: https://github.com/Oldes/Rebol3/blob/wasm/make/rebol-wasm.nest