Has anyone managed to get UART working?
It's doubtful the Serial TX Data Output pin is actually bonded to the die, like this AN8002 pic apparently shows.
You can look for a substrate diode reading, or scope the pad as another way to see if there are signs of a connection. I did not find one.
EEPROM location 0xFD (stock is 0x82) can be programmed for some options:
"V1.9 Add UART upload option: 24C02 FDH.6=1 select 9721 compatible format, FDH.6=0 (dtm0660L version), FDH.5=1 send 14Bytes, FDH.5=0 send 15Bytes (dtm0660L version)" refers to bit 6 and bit 5.