Well, it was late last night when I posted, so I didn't think about doing further research once I had that piece of information. Getting just that was a collosal frustration. It looks like WinXP (and prior OSes, probably) use Path MTU Discovery, which means that it finds MTU sizes for each individual host it communicates with. This isn't really necessary, however, as communication can go on without that by just letting the intermediary routers fragment the packets as needed. It's slightly more efficient to do PMTUD, but not by a whole lot, probably doubling packet overhead on the receiving end at most. (Packet overhead is something like 56 bytes per packet for TCP over ethernet.)

So I haven't tested this yet, but I'm about to. Cross your fingers for me.