Hm. Just spent some more time reading about tandem drivelines and kidback setups:
http://www.bikexprt.com/bicycle/tancrank.htm
http://sheldonbrown.com/tandkids.html
So I guess the usual kidback arrangement, if there is one, is to add an extra chainring on the port side of the stoker crank on a crossover-rear-drive tandem driveline, and chain that to a kidback crank with the ring on the port side. Sheldon also suggests, if the bike is to be used exclusively as a kidback, ditching the normal stoker crank position entirely and running the kidback chain diagonally to the port side of the captain's crank in a crossover-front-drive configuration.
I'm not noticing any explicit mention of how to put a kidback on a bike equipped with single-side drive, however. This could be a slight problem, since the bike I'm considering is equipped with both single-side-drive and one-piece cranks ...
... plus, the default DIY arrangement in John Allen's article linked above, using two complete solo three-piece cranks instead of a dedicated tandem set or mix-and-match arrangement, is also single-side drive.
I'm kind of guessing at this point that, especially in a fulltime-kidback arrangement with nothing in the stoker BB shell, it may be possible to convert to a single-side-front-drive arrangement, and run the sync chain directly to a kidback chainring on the starboard side of the bike ....
Actually, wait a minute. I'm an unobservant cretin. A closer look at these pictures ...
... make it pretty clear that at least one single-side-front-drive setup was not only constructed but also taken on tours. Not sure how available the exact parts used there actually are, but I guess that kind of answers my initial question!
(Obviously, it's supposed to be pretty easy to convert the Schwinn to modern cranks and use one of the recommended crossover drive arrangements, but that level of effort and extra $$/$$$ would make it silly to start with an outdated bike for cheap in the first place.)