I fixed the lost-art issue without having to wipe and resync, but it was pretty tedious.
The problem appears to be in the spec I had for naming the cover art: it was just ".jpg" with no filename. Windows XP treats a file like that as Hidden, and that must have tripped up the scanning when the iPod db was rebuilt. (MM continued to display the cover art with no problem.)
It was easy enough to rename the existing files to "cover.jpg" and then rescan the library, but every one of those tracks had two art entries: the old one, pointing to a now-nonexistent file, and the new "cover.jpg" one -- which was entered as "other" rather than "cover". I couldn't figure out an automatic way to eliminate the bad entry and re-categorize the good one, so I've been doing that manually:
Shift+Enter, Alt-T, Alt-R, tab, tab, C, Enter
over and over again. And I'm sure I've been making some mistakes, like only making that change for a single track rather than the entire album. TEDIOUS.
But after doing this and resynching, the album art appears again on the iPod.