It took some trial and error, but I manage to burn a working copy of the Y2K BIOS onto one of the 64K EEPROMs using a Realtek PCI network card (it has the RTL8139C chip).
I'm using the EEPROM in a Realtek RTL8019-based ISA card in my IBM 300GL which normally only handles drives up to 8GB in size. I had to muck with the card's I/O settings a bit to find open resources, but now I have the computer booting with drives up to 32GB in size. Perfect for me; in fact, I plan to mostly use 16GB SD cards with it.
There is one weird thing with the IBM, though. It now halts the boot sequence (both hard and soft) with an error about a configuration change regardless of how I saved BIOS settings on the previous boot. The computer continues to boot properly if I exit the error screen, but it is very annoying.
Update: I also found out this BIOS breaks the computer's ability to boot from CD. I wonder if XT-IDE would have the same effect (I haven't had time to prepare another boot ROM). It's not a big deal since I have a Gotek in the computer, but I do find CD booting a little faster than floppy.