Inversing a garbage matte is a feature I would like ( I have put in a feature request). It is handy if you wish to keep the “outside” of the garbage matte to create a frame for an insert. eg a TV set or a billboard. The problem with workarounds is that you need to “draw” your garbage matte on the actual clip eg the TV set, not on the clip image that you want to insert.
It is available in AE ( as an Inverse Check box)so would seem simple enough.