My thanks to all who helped out with my oil buzzer problem.  I believe 
we have reached a solution.

Scott Daniel wrote:
> Why has someone not been able to put an actual oil
> pressure gauge on, even temporarily ? thus preventing
> all this 'trying parts'.

David Beierl wrote:
> Somewhere in all this please tell me you put in an
> actual oil pressure gauge, either for test or permanently? ...
> Ok, I know you didn't, but please do.

Hmmm... diagnose oil pressure problems with an oil pressure gauge.  
Pretty radical idea, but worth a try.

It's an excellent suggestion, of course, and, based on David and 
Scott's advice (and that of others), I planned to try it next.  I'm not 
sure I'd ever even seen an oil pressure gauge before until yesterday.  
I had, though, looked into Van Cafe's $170 Vanagon oil pressure gauge 
kit in the course of researching this problem.  Between the cost and
the hours needed for installation, though, I was reluctant to go that 
route.  That's why I tried the cheaper potential solutions - about $10 
worth of pressure switches and an oil & filter change I needed anyway - 
first.  Many people have apparently had success with those.  (The 
pressure switches turned out to be expensive in terms of labor, but 
when I started I had hoped they wouldn't be.)

So I checked out Autozone yesterday, and they had a cheap mechanical 
oil pressure gauge meant for permanent installation, but I figured I 
could use it just for testing.  I think I still needed to find some 
fittings that they didn't have.

I also wondered about where to attach it.  It would be almost 
impossible to replace the high-pressure switch with the gauge - it's 
too hard to get to.  Would replacing the low-pressure switch really be 
useful, since my problem was at the high-pressure switch?  I assume the 
pressure is different in different parts of the oil system.  Is it the 
same at those two points?  Would measuring at the "wrong" one really 
tell me anything?  I hunted for oil system diagrams and found none.

But now I believe I can get my $17.95 back from Autozone, because:

Gabriel Hourtouat wrote:
>> actual low oil pressure...
> (...caused by a pressure relief valve that didn't
> seat properly when coming off of high engine rpm in
> hot weather?)
> The fix for an intermittent oil pressure alarm (buzzer
> plus light) on a 1990 Westy (as described above), was
> to replace the spring on the oil pressure relief.
> Can't remember where the spring resides.  I remember
> only thinking how simple the spring replacement had
> been.

I remembered the spring had been mentioned in Van Cafe's excellent oil 
pressure light/buzzer article at
http://www.van-cafe.com/home/van/page_15_9/oil_pressure_problems.html:

> There are many things that can cause low oil pressure
> in an engine: worn main bearings, faulty or worn oil pump,
> worn pressure relief springs, obstructions in the
> lubrication system, to name a few.

So I started going through the archives on the Vanagon mailing list and 
TheSamba.  Sure enough, I found multiple references to symptoms similar 
to mine that were resolved by replacing the spring, which tended to 
shorten over time.  In particular, there's a nice picture of worn and 
new springs here: 
http://www.thesamba.com/vw/forum/viewtopic.php?t=264817&highlight=happ, 
along with some discussion of why the spring shouldn't cause the 
problem, but might anyway.

Hours of gauge installation, or 15 minutes to replace the spring?  You 
know I chose option (b).

According to 
http://www.thesamba.com/vw/forum/archive/index.php/o-t--t-300493--.html, 
the spring is supposed to be 62.3 mm long.  I read in various 
discussions that sometimes the piston that sits on the spring is goopy 
and stuck, and requires a certain amount of heroics to get out.

Happily, that was not the case with mine.  The plug unscrewed easily, 
and the spring and piston plopped into the waiting oil pan, as the van 
began to drain all its oil very, very slowly.  The parts were clean and 
slick, but the spring was only about 60.8 mm long.

Could 1.5 mm really make a difference?  I wouldn't have thought so, but 
what do I know?  The local VW dealer needed a few days to get a spring 
in, so I decided to try something that had been suggested in the 
TheSamba discussion, and temporarily slip a washer under the spring to 
tighten it up a little (a little too much, really).  I then drove to 
work - highway, 90 degrees, air conditioner on full - with vague 
visions of my engine exploding in the back of my mind.  ("Well, Mr. 
Sittser, it seems the cause of your engine exploding was this 5 cent 
washer which prevented the oil pressure relief valve from opening, 
causing a fatal build-up of pressure."  "How strange.  I have no idea 
how that got in there.")

Anyway, the result was success: no buzzer or light.  This was only a 
single trial, but the problem's been pretty consistent, so I'm 
reasonably confident that this is the solution.  I'll replace the 
washer with the new spring as soon as it arrives.

Again, thank you all for your excellent help and advice, and in 
particular to Gabriel, who found the solution.  Oh, and I'm still 
curious about the pressure gauge hook-up (my questions above), if 
anyone wants to enlighten me.

            - Steven Sittser