[saurik]

FWIW, while you are probably correct in the context of Loom--a specific implementation that I honestly haven't looked at much--you shouldn't generalize to "green threads" of all forms as you not only can totally implement this well but Erlang does so: as you are working with a byte code and a JIT anyway, you instrument the code to check occasionally if it was preempted (I believe Erlang does this for every potentially-backward jump, which is sufficient to guarantee even a broken loop can be preempted).