It should be rather easy - people have succeeded using Emscripten on Nim generated C code. (Emscipten can also build WebAssembly.)
def's nimes (https://github.com/def-/nimes) is compilable by Emscripten. It doesn't use GC, but adding GC support should be easy - WebAssembly doesn't support scanning stack, but we can simply choose to launch GC only every frame (when the stack is guaranteed to be "empty").