I would love that feature, too. The aftermarket chargers will technically work. But either you can try to squeeze the actual coil plate into the device which will probably hinder the back cover from completely closing.
Or you can stick in on the back side of the device. But without a bumper sealing everything I guess that would be uncomfortable every now and then.
In addition blocking the port may sometimes be a problem. Leaving it plugged in all the time might on the other hand reduce the lifetime of the usb port due to the frequent movements from your environment.
Personally I think a coil integrated into the stock phone would be the only nice solution. When I reached out to Purism via Email I was told features like that weren’t a priority. However I do not consider wireless charging to be a fancy lifestyle feature, but simply as a very practical feature. I have chargers at home, at work and even my car has one that additionally provides radio enhancements through the car’s antenna.
And you get an enhanced lifetime of your cable charging port on top. USB C may have an increased number of pluggings I still have doubts about that.