Hum,digging a bit deeper I found this on the Metanoia website
MT-G5321 SFP is one of the Metanoia PHY SFP product series complying with the ITU-T G.9701/ G.9700 G.Fast standard which can provide up to 1Gbps data rate. This G.fast SFP can be easily plugged into any CPE Gateway which has an SFP cage on board. In order to Identify G.Fast SFP after insert to SFP slot, MT5321 G.Fast SFP will follow SFF-8472 first 128 byte format let Host to read these 128 bytes information prior to loading the program into SFP device through 2-wire I2C interface.
Which suggests that the host needs to upload firmware into the SFP once it has been inserted. That might well explain the need for OS updates on the Cisco, and is as I understand it a departure from the VDSL version of the device which does not require any such requirement for a firmware upload.
So to support this on a Ubiquiti we would need to extract the firmware from somewhere and write a program to upload it to the SFP. Some diggging around and I have found what I think is a download for the firmware used by the Swisscom box. Time to do some digging around in that and see what we can find out.
How easy it is to write a program to upload the firmware on a Ubiquiti router is another question altogether.