I recommend having a look at the source code for ScummVM. The original SCUMM engine, first used in Maniac Mansion, implemented its own VM with cooperative multithreading, way back on the C64.

Basically, the game ran "scripts" (containing custom bytecode), each given its own "slot" with space for locals. Each script is run in turn, and each script must voluntarily relinquish control when appropriate.

Of course, this is a bit more advanced than simple coroutines. If you're going to the effort of creating your own embedded scripting language, you may as well an existing language like Lua.

Example code: https://github.com/scummvm/scummvm/blob/master/engines/scumm...