A custom icon for .pyd files doesn't require a shell extension - it only really requires the icon.

I don't want to use the same as .pyc, since that has the implication that it can be deleted safely, or .py, since that can be double-clicked. So we'd need something new.

The rest of the request requires exposing new public APIs in Windows so that we can load executables from arbitrary memory locations. That's not something I can fix here :)