Thanks for reminding me about this - haven't found a solution but I just came across this: webcamoid.
It claims to work with Skype on Windows but it's not working for me with either 64 or 32 bit versions on Win 10.
I'd like to find a library that would allow me to pipe video into Skype or Google Hangouts. On Windows, that might be a DirectShow filter - I think it requires a kernel extension on Mac. I was hoping for clues in the webcamoid code, but it's all built in Qt which I don't have experience with.