I have two ideas, but no idea if either is remotely feasible:

1. Replicate the launcher's selection logic in multiprocessing and
   avoid the intermediate step via sys.executable. Perhaps put it
   into pythonXX.dll or export it from the launcher executable so
   the C implementation can be shared by both users.

2. Handle the command line argument(s) that control how 
   multiprocessing pulls the handle from the parent process in the
   launcher and perform the operation so the sub-subprocess can in
   turn get it from the launcher.

I had a third one, but it was so crazy I must have preemptively forgotten it.

Finally, I do not think this is a regression in 3.7 (although I have not tested it with anything earlier), but unless the implementation of multiprocessing has changed radically in 3.7 I cannot see how it could have (not) worked any differently before.

On the other hand, venv is essentially the "official" way of installing Python applications these days, so the issue should have a high priority for fixing anyway.