Is there any way to make an old Powerbook G4 into a field monitor...not using Firewire?

I don't believe there's any way to get analog video into a powerbook unless you use some sort of external capture device, which would require firewire. Perhaps there's a cardbus solution but I've never seen one.

Software like BTV pro can turn your Mac into a monitor using firewire from a DV camera, but the quality is somewhat limited.

http://www.bensoftware.com/btvpro.html