Wow.
I've never had such a quick & full response to anything I've posted
here.
Thanks to all the creative volks who seem to have BTDT!
First, a little history: I got the van in May and drove it home from LA.
Once here, I had to R&R the rear bearings, which led to a CV
re-packing, which led to a gear oil change...
Of course, I tried to get the fill plug off first, but it just wasn't
happening. It seemed to be stripped, and I broke off a small chunk when
I tried removal.
OK, I coulda stopped there, but I'd bought the van from my
brother-in-law, and while he is just plain scary-intelligent, I had a
bad feeling about the tranny lube.
So I drained it anyway (sorry Bruce)!
Glad I did- the stuff that came out must have been the original,
prototype gear oil used in the VW38, and half water to boot!
 From there it was simple math to figure out how much I had to get back
into the tranny, and I nearly destroyed one of those $9 hand pumps
doing it. I had to stuff a rag around the hose going into the drain
hole, and then do some fast hand work (no jokes) to get the plug back
in, like Mark suggests.
So going forward, I'm going to pull the tranny, then heat the area
around the fill plug, then put on the wrist bracers I wear with my
renaissance fair costume, then use vise grips to twist that sucka outta
there.
Failing that, at least I'll have room to get an easy-out in there at
the right angle.

I had initially wanted to avoid yanking the tranny because I'm
ostensibly only in this for the engine swap. So far I only have the
engine removed, but I've also:
-started to relocate one of the batteries
-finished ripping out what was left of the A/C system
-put in new cup holders
-started wiring in headlight & auxiliary backup light relays
-removed an old air shock system
-and dicked around with the tranny. Gonna replace the bell housing
gasket while it's out, I guess...

Anyway, again, thanks to everyone who replied! I'll post results when I
have 'em.
-Simon