Ok, so not being able to resist figuring this out, I now have been able to flash my U14 EEPROM with changes needed to allow the 12 GHz prescaler to work without a scale value being needed. The earlier posted instructions are probably correct, you WILL need a copy of U14 that starts with "HP53131 is a winner" near the beginning. Mine did NOT have that and/or whatever other bits are needed, despite having added the ones listed in the earlier post. To get this working, I added a U14 socket to a counter that already had a factory 12 Ghz prescaler, read out the chip, and then burned one for my other counter. Of course this then needed a quick recalibration.
I guess when the 10 GHz one that I ordered shows up, I will sell it or stick it in a 53181a that I also have.