After finishing the MMC6 PRG-RAM, I checked NesCartDB for other mappers with battery backed PRG-RAM.
Mappers 19, 80, and 82 may have battery backed PRG-RAM. I wrote support for Mapper 19 SRAM but I didn't know about the carts that use the internal PRG-RAM.
I'll look into adding support for the battery backed PRG-RAM for mappers 19, 80, and 82.