I came up with another idea: Why not use a proxy that redirects traffic from Alice to your host, does the authentication there and then sends an ok to your software on Bob's machine? You would also end up with a lot of traffic which is probably not what you want.
I don't think you will get out ouf changing you webcam software, maybe by adding a authentication module that will be started whenever someone tries to access the script.
I hope this helps

Regards,
C-Keen