I briefly thought about doing that, but came up with the same question!
That setting tape is less than paper-thin. If you fill the gap with black RTV and firmly shove the glass down in until it doesn't want to go any further, it pretty much self-centers. Allow the excess which squeezes out to set completely for easy cleanup with a razor blade.
One could argue about whether it actually self-centers or not, but what is the worst case scenario? Off by 1/64" one side or the other?
And then one needs to consider, so what if it is off-centered? The vent window is sealed by the rubber contacting the frame not the glass.
