It runs!
I finally rode it last night. The derailleur is off right now, and I am just running it in 1st gear only. IT IS FAST! I still have a few details to work out, but the major engineering obstacles have been resolved. I'm sure adding the derailleur will be the final challenge, but I need a pull starter before I can do that.
Gearing on these bikes is universally measured (by us) by teeth on the rear sprocket, so here it is: 1st gear is 32t, but the jackshaft has a 1.17 (21/18) ratio, so the effective 1st gear is actually equal to 37.44t. 2nd gear is 24.75t (21t actual), and 3rd gear is 21.06t (18t actual).
I don't know if 3rd gear will be useful, but it has a good spread from one gear to the next, and I can always lower the whole thing by adjusting the jackshaft. If I change the jackshaft to 21/15, I would get 1st=44.8t, 2nd=29.4t, 3rd= 25.2t.
I will post photos soon. JFleck took a few really good photos last night after the test run.