I carefully
> removed the overflow tank hose and slowly opened the expansion tank
> pressure
> cap, hoping to depressurize the system to avoid damage and more boiling.

Actually, pressure is your friend in cases like that. Pressure raises the
boiling temperature. Ideally what you want to do is leave the pressure cap
on until the engine cools and the pressure is equal to outside.

>  Coolant level is stable and I'm hoping that
> I got very lucky in not breaching the head gaskets internally or
> externally.
> I'll pressure test the cooling system at earliest opportunity. Any other
> testing suggestions, other than empirical observation, to confirm that I
> didn't harm the engine?

Given that you shut down immediately upon loosing the alt belt, the engine
didn't have much time to build up a lot of excess heat. A little localized
boiling probably didn't hurt anything, so long as the engine wasn't
overheated. Not something you'd want to make a habit of, of course. What
really gets you is the major steam cavitation that results from running the
engine up to high temperatures and THEN shutting it down and letting it boil
like crazy, like the PO did repeatedly to my van because he was too thick to
replace the clogged radiator. But even then, my head gaskets never had more
than a very minimal ooze. What ended up killing my original engine was a
broken head stud, weakened by corrosion caused by the PO refilling the
cooling system with nothing but plain tap water every time he overheated it.

Other than that, a great trip, although my wife declared that we will never
> again cross the Mojave without a functioning A/C.

Funny thing, my wife said the exact same thing last month after we drove
from St George to Los Angeles in the 105degF heat. I'm not sure how much use
the AC will be when urging that poor wasserboxer up some of those long
grades though. I'm thinking I might not even try it until I've got a subie
in there...

--
John Bange
'90 Vanagon - "Geldsauger"