First, PVC is a type of Plastic, and NOT an emissions control system. That is the "PCV". Please remember that, as we are professionals and are judged by our use of words (whether we like it or not!).
Second, there is no way what we do will blow the seals. In the rare case where a leak does develop after disconnecting the vacuum, this is due to worn seals, and not pressurization of the system.
If the engine leaks oil without vacuum, then the vacuum was the only thing keeping the oil in the engine to begin with. Think about that for a minute. These kind of leaks tell you the seal was allowing air into the oil system at the crank, absolutely aerating the oil, and thus degrading it's lubrication properties.
So, if it leaks it is actually a GOOD thing for the customer to know, as the worn seal will, of necessity, cause accelerated wear. No way around it. The customer should be glad to be able to identify this issue that it be corrected without further damage to the bearings et al in the engine.
All mechanics, not understanding this simple fact, will speak negatively about capping the system in this regard. Take a moment and walk them through as I just did and (if they're not too arrogant to learn something) will thank you for the better understanding.
This can also be used by the average mechanic to bring in more business, if they're the business-saavy kind!