The USB-on-the-go (OTG) webcam idea is dead in the water. The problem is that when the webcam is plugged in, it draws power from the phone and the phone just cannot output that much to make the webcam functional.
There are USB OTG "splitter" cables, that split the data into one cable and the power into another, and this *would* solve that problem (i.e. data is plugged into phone, power is plugged into a plug socket)...
Then I realised that there's another problem: There's no way to power the phone when something else is plugged into it. I searched around, looking for micro USB OTG cables that can simultaneously host devices and draw power, but it just doesn't exist for older USB.
USB-C I think can act as both a host and peripheral, but older USB operates under a distinct slave/master relationship.
So now I need to get creative again.