I don't know if this is kosher but I was putting a GFCI in a bell box that didn't have protection and the bottom screw was rusted in. (no surprise there). Of course it snapped off.
I didn't have a spare box so what I did was screw the GFCI to the cover via the "decora" cover screw lug and then used 2 screws to put the bottom of the cover into the unused side screw ports in the bell box. It really seemed very secure.
The top went in normally.

Sounds perfectly OK to me ... especially if the ground wire terminated on the device!

That is exactly what I'd have done. That's really not much different than a device attached to a raised cover on a 1900 box.

Reno, that was the other thing I did. I ran green wires to the receptacles. This was the last of my clean up around the neighborhood for the community assn.
I think this was the last thing that really needed fixing.
Now they might have to throw away some of the "leaky" Christmas decorations. There are no more non-GFCI outlets around anymore

Greg:
Sounds acceptable & professional!