OK... IT'S POSSIBLE !
You have to install the patched .ipa DJI Go 4.2.22 (this version, not the last one) side by side with the last stock version 4.3.x.
Switch on the bird and start the patched app. At the tweak prompt set FCC mode, boost or 32 channels and go to the fly screen. DJI Go 4.2.22 was the last version before
MAVIC 2 launch so it can't manage it. It will say that the bird is disconnected, the gimbal has errors and probably will not show any image. No problem: exit the app and start the stock version and... the magic is still there! Check in HD section and you can see that the signal thresholds of 1km and 4km are at FCC level. I already have to try it "on the field" but it seems working well.
Finally, Azuréen, it seems that...
FCC mode on Mavic 2 Pro IS possible !