Thanks a million for all the information and suggestions. It sounds like buying one of the Mahalo uke/guitars might be a real crap-shoot.
As I said initially, the whole idea is to have a small, handy instrument as a strictly practical working tool, not for performance. It should be almost as convenient to carry around as a Master Key chromatic pitch-pipe (slips into a shirt pocket), but a bit more versatile, a stringed instrument as much like a guitar as possible, and if it is too big to carry in a shirt pocket, it should still be small enough to stuff into a tote-bag (Tiny Tim style?). I want to be able to use it to pick out melody lines or try chords with—but not finished arrangements, and as I said, not to use in performance. As long as it can be tuned to a tuning fork or electronic tuner and I can play chords on it (regardless of the chord voicing—the final arrangement would be worked out on the guitar), it will do. And it should be very economical (i.e., cheap).
What I'll probably do is cruise a few used musical instrument stores or pawn shops and see if I can pick up a decent used soprano uke. A uke should actually do nicely, and that'll give me a chance to try it out first.