The leostick does appear to have some fundamental problems with the way the arduino-as-isp stuff is handled, and I think this shouldn’t be a huge problem for me now that I can get code onto the Diavolino, which ought to be much more usual in that regard.
As for the other TTL adapter, so far I know that a loopback does work as expected. There’s actually a pin on this adapter labeled “reset,” which makes it pretty obvious where it’s supposed to go, and as I said, the power is at least working sufficiently well to drive the board. I do wonder what (if anything) it’s doing with that reset pin, and perhaps I should tack a multimeter onto the end of it and see.
Luckily, I also have a pro-mini clone — the intent here is to use the Diavolino in development of a project to be built with the pro-mini — so I can verify that the Diavolino isn’t the only thing which will program with one TTL board and not the other.