I am not sure how a freeze plug would work as they pop out from pressure. Unless i was to pop them in from the inside but that is nearly impossible.
It would work fine, cooling systems average 12-18psi of pressure and expansion plugs work fine.
Non-turbo exhaust systems should never run with that much internal pressure, stock manifolds rarely get above 10psi, headers 5psi.
In a perfect exhaust design the pressure at each exhaust port should be slightly less than outside air pressure, this pulls exhaust out of cylinder faster, but that would be perfect and it isn't a perfect world, lol.
When you run from a smaller diameter pipe to a larger diameter pipe(collector) you increase velocity and reduce pressure.
So when the exhaust pulse from one cylinder hits the collector its velocity increases and pressure drops, this pulls pressure from the other cylinders pipes, and keeps over all pressure down.
The common myth about engines "needing" back pressure came from people putting in larger diameter exhaust pipe manifolds and losing power, so they think "I made the flow too good so engine must need back-pressure to get more power"
But what they had really done was to increase the back-pressure by reducing the velocity with the larger pipes.