This is a bit of a work around but we don't have any nice way of doing this at present.
Replace the relevant bits from IF-MIB with the data from UBNT-AirFibre-MIB.
Kindly had access to an airfibre unit to test this:
eth0 on AirFibre units get data from another MIB
Auto-Deploy finished, Test PR at http://2544.ci.librenms.org or https://2544.ci.librenms.org
That's definitely a bit icky...