Hey guys, sorry for bumping this thread, but I think I just cracked the case.
The FM18W08 is the chip you want, should work on any cartridge that has SRAM.
If you are using MBC3, connect the FRAM CE pin directly to the cart CS pin (pin 5) instead of the RAMCS pin on the MBC3. Leave the RAMCS pin on the MBC3 disconnected.
If you are using MBC5, this chip can be used as a drop-in replacement with no additional work.
ONLY ON MBC5:
The power-down chip can be removed if you place a 10K resistor between the Vcc and CE pin of the FRAM.
EDIT: Unfortunately this method only works for Gameboy Color, not DMG. Games on the DMG do not boot if RAM isn't connected to the MBC3 RAMCS pin. I have a feeling it has to do with the GBC running at 2x the speed, but I can't say for certain. This will take more work to figure out.