Hmm, so you are.
I don't think the ctypes library supports it.
In visual basic years ago, we used to create the transparent area in script merging multiple regions, but it wasn't advisable for complex shapes because of the resources it used.
I'm sure Rojo will get back to you with something, but in the meantime take a look at the WSO library.
http://www.veretennikov.org/WSO/Help/ht ... 8fdd5c76c2
It has all the usual functions for regions, like ctypes has, but take a look at the bottom one.
It allows the use of a mask, and it's Python compatible, so it should do what you want.
I haven't used that library for that purpose, so I'll be interested to know how it goes.