> I assume you wouldn't expect to support shebang lines 
> like "#!python3.6r"?

That's already supported in py.ini in the [commands] section, per PEP 397. I've used this from Vinay's initial releases on Bitbucket, before the launcher was officially distributed with Python. 

One catch is that commands can't start with "python" because that's special-cased in the launcher. Instead you could use the following:

    [commands]
    rpython3.6=F:\Python\dev\36\PCbuild\win32\python_d.exe

Then create a script with the shebang `#!rpython3.6 -i` that starts the REPL after it executes.

What's missing is a way to run custom commands directly from the py[w].exe command line. I wouldn't want to hog a letter such as "r" that a python.exe option could use someday (noting that command-line options can be combined like `-SORE`). But maybe it could use "--" or some other symbol to demarcate the command.