since I have wireless chargers everywhere (coming from a Galaxy S6) I wanted to be able to continue using those and found a Qi charger accessory that works very nicely - Nillkin Qi Receiver (USB-C Long Version):
I installed the charger coil underneath the plastic back cover of the FP3 - yes, it barely fits (after removing the useless “Discover Innovation” banderole from Nillkin).
The back cover stands out <<1mm - it is just barely visible that the surface is not quite even any more. And with the bumper, there is no issue at all as it lifts the back cover above surface anyway.
And yes, it charges just fine. Putting the phone on a charger, it first states “Charging slowly” for about two seconds, but then switches to “Charging” so high-speed charging is apparently working.
The only tiny drawback is that since the USB-C plug is off-centre, so is the coil’s center on the backside and the phone needs to put into a slightly off-centre position on a given charging station. Not an issue with the stations I use, but some of those where you put the phone into in an upright position or tightly confined space might have issues here.