I just changed a waterboxer head for the first time (for reasons other than leaking coolant....a problem I personally have never experienced with my two vanagons with a total of 320,00 miles). There are two seals...a metallic o-ring type seal twixt the cylinder wall and cylinder head to seal the combustion chamber and a rubber seal twixt the engine block and the face of the head to seal the water jacket. The latter only sees the cooling system pressure so any increased combustion chamber pressure from the NOS would have no effect on the coolant seal.....unless of course you blew out the combustion chamber seal first. But these are pretty sturdy. 
However, having said all that, your point is well taken. 
I was only half serious.
On the other hand.......????

, Thomas wrote:
>Nitrous in a Vanagon?
>Hmmmmmm???
>Wouldn't that be a relatively easy way to get extra, albiet temporary,
>power for hills and passing?
>Has anyone ever looked into this?

I'll bet you a dollar that it would be a good way to displace all of your
coolant from the system with combustion gasses whistling around the head
gaskets, but yeah.  You could pass a half dozen other vehicles in the 30
seconds of un-vanagonical power that you'd experience before the valves
liquified and went on a vacation to your catalytic converter.  :)
But that's just my pessimistic opinion, I believe that any turbo boost of
high enough pressure to make a noticable difference to a vanagon's
acceleration would be too much for those gaskets to handle, no matter what
balance of blue goo and pure, distilled, imported, blessed holy water you
have in there. A violent combustion like nitrous oxide mixture is no where
near as gentle as a turbo booster.

Thomas Brobst
Accu-Sort Systems, Inc
Staff Mechanical Engineer
215-721-5061
fax 215-799-1600

The First Casualty of War Is Truth