Seems the upgrade url could be plugged into the "wpmublogsaction", "manage_blogs_custom_column" action hooks, or the "manage_posts_columns" filter in wpmu-blogs.php.
You know what I mean? ... If blog(spam or not) has no taxonomy tables, then show the upgrade button(for that blog alone) as an "Action" under SiteAdmin-->Blogs. Needs a flashy "I'm not ready for Tags!" spinning gif.
BTW, deleting "pre-taxonomy" spam blogs creates mysql errors looking for tables that don't exist, of course. If the upgrade is run before deleting, then no errors, but that seems kinda pointless, albeit cleaner.
Nevermind,
Just read the ticket, as soon as wp-admin is run, the upgrade happens anyway.