After poking around in the GitHub, I figured out why I was having so much trouble. Right now you need an attached beam generator set to the right wavelength in order to relay.
Specifically, MicrowavePowerTransmitter.cs:681 was using the native wavelength which defaulted to IR since it isn't set for the FELA. This value was used in MicrowavePowerTransmitter:888 to become the only supported relay wavelength in MicrowavePowerTransmitter:908. Without attaching a beam generator there is no way to set the frequency for the relay, and it will default to the active relay dish's native wavelength, or IR if that isn't set.
Attaching a UV beam generator got it functioning as a UV relay. What confused me at first was that the dish linked for relay was reporting that it saw 0/2 transmitters, but it looks like that value is nonzero only in receive mode so that part was working as expected.
Thanks for the continued great work!