It is my understanding you need to have this certification done every
24 months if you either (a) fly IFR, or (b) fly anywhere that Mode
C is required.

Sorry to make you do all that typing Ross. I actually know how they do it,
since I watched them do my previous RV-8. Unfortunately, the avionics guy
is so busy now that he doesn't make trips to the local airports anymore. We
even tried to lure him over to do two of these, but no joy. I can't even
fly the plane to his shop because it's outside my test area. I may just
have to take my transponder and encoder over to let him do it on the bench.
That would make it legal I think, but still silly.

Thanks for the comments,
Rusty