Advanced Mac Substitute uses a factored approach. 68K emulation happens in the back end, which is a collection of processes connected by Unix-domain sockets, portable to basically any POSIX system.
The front end deals with displaying graphics and forwarding user input to the back end. Working front ends include SDL2 (by a contributor), X11, Linux fbdev, VNC, and five different macOS APIs (Cocoa/OpenGL, CGL, AGL, Quartz, and QuickDraw).
The front and back ends communicate using FIFOs and shared memory. I'm aware that certain platforms will require refactoring all of this to run in a single process. If Emscripten is one of them, then it won't be as simple as you suggest.
In any case, if I were the one doing the port, I might write a bare-bones front end just for this purpose, possibly using the fbdev one as a starting point.
