As far as I know, only tape based cameras have a firewire jack. Like you said, the USB ports are only for file transfer.
If you still have a tape based camera, you don’t need to have a tape in your tape camera for it to stream. On my Canon XH-A1, and A1S under the
SYSTEM SETUP menu, chose the DV CONTROL option.
Basically, it’s the same as setting up recording to a FireStore or other external recording device through firewire. I also use this setting to record to a laptop.
I’ve had other cameras, such as an old Sony
Digital 8, with firewire output that you don’t need to even set anything to set anything.
If you no longer have a tape based camera with a firewire port, pawn shops and flea markets have tons of used ones. Also garage sales. For what you want to do, the tape mechanism doesn’t even have to work.
Roger