I only find 2 current light GA type Diversity enabled transponders.
Good thread:
http://forums.avidyne.com/diversity-antennas-for-transponders_topic1589.html
Says the Garmin GTX 330D (ES version) supports Diversity. Note the "D" in the hardware model.
L-3's Lynx 9000NGTD, "D" for diversity, or top and bottom antennae, lists for $6300 USD as an all in one solution. 1090ES out with WAAS built in, ADS-B In and Out.
I assume these 2 types would be the "hope" for a reasonable, existing solution assuming this becomes a Canadian mandate. Would possibly have to know if use of both antennae is constant and compliant with the needs of the OP space-based system.
The issue I see is I believe WRT ATC transponder/radar, the diversity is controlled to give best performance from air to GROUND. No idea what software or hardware makes Aireon "happy".
Large aircraft have had diversity for transponders with TCAS for at least a generation. I assume that the upper and lower antennae receive simultaneoulsy and the software controls reply transmissions directed to the stronger receiving antenna.
Just the S.W.A.G. of a pilot, not engineer or avionics guru. Walt?
ETA, Garmin FAQ leads to assuming they offer antennae diversity on some options. No idea about the other manufacturers.
https://www.garmin.com/us/intheair/ads-b/seven-questions/
"Why should I consider a dual (diversity) antenna installation for my ADS-B solution?
Antenna diversity technology configures your aircraft with both top- and bottom-mount antennas to reduce the potential for antenna “shading,” which helps prevent target drop out during turns and maneuvers. An optional top-mounted antenna also comes in handy during ground operations, when a bottom-mount mount antenna might not have clear line-of-sight to receive ADS-B ground station signals or transmissions from other participating aircraft."
The Aireon website does not even mention light GA.
https://aireon.com/resources/overview-materials/technical-specifications/