I was testing adding a ULA prefix in addition to the Tracked Interface GUA on an interface. I did this by adding the appropriate VIP entry and it did work. I was able to ping the GUA and the ULA address on the interface, etc.
I noticed that when I do a nslookup for opnsense.mydomain that I see all of the ip addresses for each interface, but the GUA of the router for the interface with the VIP was no longer listed. Instead, the ULA VIP had replaced it.
This may be ok, I just expected it to either still show the GUA's of the router in DNS or maybe add the ULA VIP in addition.