I would like to add an external high precision reference frequency for my FT-857D. There is a TCXO option, but nothing that could be disciplined by GPS.
My idea: to integrate a clock detection/switching in a PCB board that will detect if an external clock is available and switch to that, else keep the clock provided by the TCXO. Think about a standard TCXO, available as replacement for the Yaesu TCXO-9 board, but with a clock detector, a pigtail SMA connector and a clock switch:

This is the general schema.

ul.fl to SMA pigtail (10cm):
ASIN: B0B9RXDLNN