Hmm.. what do you think about "/dev/videoX", "/dev/video(X+32)", with X ranging from 1 to 32 reserved for real devices only.
Sounds good to me. (But will reduce number of possible real video devices by half.)
Well, I don't know if it is possible to do that without interferring with the v4l framework.
You may try to use a dynamic device number range. This will work fine with udev, avoiding to limit the device range on V4L.