I think the tail will work fine. My GMU-44 is going in the wingtip (Garmin 900X, not the G3X) mainly because an RV-10 in Sedona had a lot of difficulty passing the certification due to the elevator horns and rudder cables. They ended up having to de-gauss the cables. Moot point because Garmin no longer requires the post-installation for the 900X (and never required it for the G3X as far as I know). From what I have heard, the Garmin certification is way more restrictive than it needs to be for our purposes.
Oh, I talked this over with Stein and Aerotronics both (Stein was nice enough to contribute a mounting bracket for the wing location; many thanks!) and the consensus was that the trim motors are not an issue; ferrous material is much more of a problem.