Threads abound recounting the horrors of pressure and carboys. There was another one going here in the last week.
I'd be leery of doing it with a glass carboy, but looks like a great solution for a plastic fermenter. I use Speidels most of the time but don't have the space in my fermentation chamber to use the Brulosophy (Bruloon?) mylar balloon trick
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If you put even enough pressure on a Speidel to initiate a closed transfer (1-2 psig) you can see it visibly balloon out, to the point where it rocks around because it no longer has a flat bottom to sit on. Good illustration of the amount of force being put on any fermenter.
With a properly filled S-type airlock, suckback is impossible, although air will be drawn in.
If height in a chamber is a problem with the Brulosophy balloon method, it seems the OP's idea of a length of tubing jammed in a stopper could allow a balloon to be placed where space allows.