How to reinstall Magic Trackpad foot?
Right, all the useful info is...

https://discussions.apple.com/thread/4646963 (but the thread is closed)

https://www.ifixit.com/Guide/Magic+Trackpad+Rubber+Replacement-Reinsertion/16825

I may have just fixed this...

I gently lifted the white rear away from the metal case with a tiny flat-head screwdriver, from the side. You need to be careful not to break off the catch that hooks the white rear to the metal case at the bottom end of the device, above the hole pictured.

I put the button bit back in the rubber foot and was able to push them through the opening with the same screwdriver, back in to the hole and let the white rear go back over it.

This was without really lifting the white rear catch out of the metal case.