I think this is best written as:
* extend py.exe to handle PEP 514 properly
* write "PythonCore\dev36" (or equivalent - maybe "PythonDev\dev36"?) keys on each build

That way you could use "py -dev36" to get your latest build from source (actual tag open to bikeshedding).

Right now we write "python.bat" in the root of the source directory for the most recent build, so maybe that's a good enough workaround?

While I'd like to enable a "register" command, it's complicated enough and I suspect of limited value compared to directly editing the registry (or updating other interpreter's installers) that I'd rather not worry. A big part of the reason for PEP 514 is to be able to point other interpreters to it and say "do this and your users won't need to know where you installed everything".