> We need a way to allow multiprocessing to spawn the real python.exe instead of the launcher executable that's set as sys.executable.

Got to a computer and had just reached the same conclusion.

Given the environment is inherited, it's easy to do:

>>> import _winapi
>>> import multiprocessing.spawn
>>> multiprocessing.spawn.set_executable(_winapi.GetModuleFileName(0))

This may interfere with others who currently override sys.executable when hosting Python, since they'll get their override by default. But changing sys.executable to _not_ be the venv one will break anyone who doesn't correctly inherit environment variables (specifically, __PYVENV_LAUNCHER__, but this is deliberately not documented ;) ). 

Perhaps we just change the multiprocessing default on Windows when sys.base_prefix != sys.prefix?