Supporting PEP 514 tags is an interesting idea, but maybe you could go into more detail about what you have in mind. Are you thinking of using the SysVersion, SysArchitecture, and [Windowed]ExecutablePath fields where available instead of parsing version tags and assuming "python[w]/exe"? How would the expansion of candidate installations affect virtual "python" shebangs and command-line arguments such as -3.6-32? If there are several 32-bit 3.6 installations, which one wins?

The PY_PYTHON environment variable could support qualified tags such as "PythonDev/3.6-32". However, arbitrarily named tags can't be supported on the command line without introducing an option reserved for the launcher. It could look for an `X PY_PYTHON` option such as `py -X PY_PYTHON=PythonDev/3.6-32`. That's a bit verbose, but simpler than `cmd /c "set PY_PYTHON=PythonDev/3.6-32 & py`.