My engine also had a closed cooling system, and it would "puke out" roughly a gallon and a half of coolant when I was getting boost under load. Short answer: yes, it happened to me (not sure if it was the cooler - happened shortly before I got a worn-out engine replaced, the cooler was the likeliest suspect, and was replaced as part of the engine). So they instead just used an off-the-shelf expansion tank same as other FWD VAG products used, with a special Vanagon-only vented cap that went over to the extra overflow tank. * Warning: useless bit of Volkswagen trivia: in case you were wondering, the reason why Vanagons DO have an overflow bottle alongside their expansion tank is because the Vanagon's cooling system is so HUGE with so much volume that they would need an expansion tank twice the size.
Not like the vent on a cap on most other cars that allows the pressure to bleed off through a small hose to an overflow bottle when the engine gets hot, and then allows it to be sucked back into the engine from the bottle after the engine cools off. The "vent" on the bottle is just there if the pressure should get TOO high, as a safety valve. That is why the expansion tanks are never supposed to be totally full, they need some air at the top for expansion and contraction.
Regarding the pressure in the cooling system, with the exception of water cooled Vanagons*, water cooled Volkswagens have a closed cooling system, there is no overflow tank that is vented to atmosphere.
What "problem" are you having (or think you are having) that prompts this question? There is no "pressure" in the exhaust of the EGR cooler that would overcome the pressure in the cooling system, just enough pressure to overcome any positive pressure in the intake manifold. The only way I have ever seen an EGR cooler fail is it blew coolant into the intake and it did so pretty suddenly and pretty bad, like a Batman smoke screen bad.
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