I added wp_mail() to wp_cache_gc_cron() last night as suggested and had plenty of e-mails when I checked in this morning.
I've noticed that if I haven't checked it for a while, the options page for the plugin takes a long time to load and often hangs after writing the page down to the Cache Contents (i.e., the page from Cache Contents on does not appear). The only remedy is to delete the cache using the dashboard link. That then hangs, but after canceling the page load and reloading the plugin options page, the Cache Contents show that it was successful.
The long loading time occurs with my less-busy installations as well (especially since I check them less frequently). And I notice that manually deleting expired files doesn't touch the super-cache expired files and also deletes the cache files, both legacy and super.