So I went ahead and gave Plan A a try. I found a 1 1/8 threaded-to-threadless converter at a bike shop for $11 CDN. I explained to the bike shop guy what I was doing and he stared at me aand blinked like I had walked in and told him that I was the one true Messiah here to perform my first miracle by shitting out pickles and sausages for his lunch.
Then he started giving me the usual bike shop guy dogma of "I know it can't be done because I've never done it." "Your steer tube is too short. You must buy a new fork. BUY A NEW FOOOOORK." Whatever. He reluctantly sold me the bit. I think I shattered his belief system or something because when I left he was on the floor in the fetal position, whimpering that my steer tube was too short.
So, to recap - the problem: find a cheap, simple and effective way to deal with short steer tube.
How I did it:
I started with the stem on tight and adjusted in the usual way using the top cap & star nut. I then removed the top cap and drove the star nut through (stem still bolted tight). I then loosened the stem (being careful not to bump the fork and loosen the headset) and dropped the converter in. Tighten up the converter, re-tighten up the stem, done.
Well, almost. Turns out the converter was actually a bit too big. It fit nicely in the 2002 Z-5 I had taken off the bike, but the 2002 EXR has thicker steel in the steer tube (hence I'm not worried about the converter splitting apart the steerer). 5 minutes of lovin' with a Dremel tool, and it slipped right in.
The Pros:
It works. I now have a solid, trustworthy connection between the steer tube and stem. And that's what this was about in the first place, right?
Cheap and easy like your mom. $11 and 15 minutes work with no special tools. Much cheaper and easier than a new steerer, new fork, grinding head tubes or shelling out for low-stack bits.
The Cons:
Lack of in-the-field adjustability. There's no top-cap and star nut. Also, when I overhaul the headset I'll have to go through the process again, but it's really not too bad.
Weight. I'd guess that this thing weights about 250 grams. Norba racers who happen to have shortish steer tubes won't be lining up for this one.
Looks kinda funky. Without using spacers, it looks like I was too lazy to cut the steerer of my new fork; the top cap is about 1.25" above the stem.. Any longer and it would like Mr. Gerretson's unicycle from South Park. On the other hand, I have lots of height adjustability if I do use spacers.
The bottom line: I'm satisfied and confident that it works. Someday when I find another smokin' deal on an 80 mm fork with a long enough steer tube, I'll swap it out - but for the time being this is a workable setup.
Thanks again to everyone for their feedback.