So, I bought one of those wooden-bead seat covers. Seems to do the trick.
It's not perfect - it's not like I have an air-conditioned mesh back or anything, but it's at least as good as the cloth seats. As an added benefit, the narrower strip of beads that runs up the back of the seat reduces the bucket shape of the seat, an improvement for wide-shouldered folks like me. The only drawback I've found is that the beads do lift me up about half an inch or so, reducing my head-clearance and forcing me to put the seat in its lowest-possible position.