Not too much help for those in the boondocks, but where I live and
fly, there are sufficient overflights by txp-equipped GA in touch with
ATC or TCAS to give alerts when my MRX is on the ground. So I can
check it at home or at the gliding site.

I suggest others check this way before flying with it, if the
situation is suitable.

Thanks to Bruno for the heads-up.

Chris N.