The question I was getting at with the original post was simply what would happen if you just hook up two OS 9 machines with a FireWire cable and boot them normally. I was wondering if that arrangement had any risk of damage/software corruption, or if it would even work as a network connection. As mentioned above, straightforward networking with Ethernet should work (something I don't want to do in this case for a variety of reasons, ranging from speed, various inconveniences, to the bad engineering of the ethernet jack on one of the machines), as should FireWire target disk mode, which is what I'll use.
As for FireNet, it's not clear from a quick check that the OS 9 version of that is still around, but thanks for the suggestion. In any event, it should not be necessary. Target disk mode should work for my purposes. I'm just not going to do the experiment of hooking both machines up and booting them normally.
PowerBook G4 500 MHz Mac OS 9.2.x