sounds like a good idea. are they easy to pop out/in? this is my first T4 and not had much experience yet, swapping out bits and bobs...Just replace the glass, they are about £12 each. They fit on a large socket type push-on fitting and thro age, they become loose and vibrate. I have replaced both of mine with no problems. Ring GSF for the mirror glass with backing plate.
In my opinion using anything to pack the mirror is just a tempoary bodge, just not worth it when they aren't expensive new T: