I have a GoPro and really want to make this work as my webcam.
You may have a look here.
In that thread the possibilities of controlling an external camera with Octolapse is descibed.
Hi @epiic! I'm glad you liked that video
In that video the gopro is being used to take snapshots, not as a webcam. It is possible to use the gopro as a webcam, but it requires additional hardware and quite a bit of setup. You'd have to be REALLY careful to get a capture card that will work with your environment (OctoPi maybe?), and you'll have to customize your mjpegstreamer to use the new hardware.
Looking at the cost of those usb capture cards, I'd probably recommend that you get a high res webcam instead.
I hope that helps somewhat.