I called them and had the case escalated to Senior Tech Support, who handed it off the Engineering. I'm supposed to hear back from the engineers within about a week.
I managed to have Music.app stable long enough to load my favorite Smart Playlist and download all the tracks in it. I then turned off iTunes Match and the downloaded songs remained available and the Music app, which is now usable, though no playlists are visible so I have to search by artists, song, or album.
I suspect the problem may be with bad metadata or excessive library size/insufficient processing power for a library of that size (my library is over 22,000 songs, which is under the 25,000 song limit of iCloud, but may be too much for my iPhone 4).
Things I have not yet tried:
- creating a new iTunes library with only a few songs or albums that you know are properly tagged and trying to sync that to iCloud
- syncing my iTunes Match library with an iPhone 4S