So there are two fixes that make the site work:
1) make free users on Chef read-only, to eliminate read/write lockiness in non-concurrent MyISAM tables.
2) block the backup clients for people on the Chef cluster, and resume read/write access to all users.
For now I've gone with #1, since I have to run to dinner because I'm starving and I need the site to work.
Perhaps later we'll do #2 for a couple hours while we rush a migration of all those users on Chef to InnoDB database clusters.
But this is just a warning that we'll only be shutting down the archiver access later for that one cluster (like 7% of users?) and just until we finish migrating people to better hardware.
But for now, sorry free users on Chef.... for the good of the many? :-/
It was the best I could do for now....