This sounds like 2 related items (but I'm happy for them both to be under this issue, I'm not suggesting we need 2 issues):

1. Allowing py -register to add PEP 514 metadata to the registry for the given path.
2. Allowing py -<tag> to use PEP 514 metadata more generally, rather than its current explicit registry scan.

I'm OK with both of these in principle. I'd suggest that (1) register under HKEY_CURRENT_USER (so that it doesn't need elevation, and so it's a per-user setting) and that it uses the PythonCore "Company" value (as it's a core CPython facility).

We might need an "unregister" option as well, to avoid leaving clutter in the registry, and maybe even some means of listing what's been registered. That may be overkill, and manual registry tidy-up may be fine (but the same could be said of -register).

There are other considerations, too - this should probably be solely for interactive invocations. I assume you wouldn't expect to support shebang lines like "#!python3.6r"?

I'm not likely to have time to work on this myself in the short term (my C skills are a bit rusty, TBH) but I'm happy to review patches, etc.