Well this leaves me looking pretty silly.
I don't run MySQL on my local system at all. :) I've upgraded to Virtualmin 3.44, and went into module config, chose to update multiple MySQL servers (listed them space separated: 172.16.10.30 172.16.10.32 172.16.10.33), and then tried enabling MySQL on one of my user accounts in Virtualmin:
Creating MySQL login .. .. MySQL database failed! : SQL insert into user (host, user, password) values ('localhost', 'numbski', password('xxxxx')) failed : Duplicate entry 'localhost-numbski' for key 1 at ../web-lib-funcs.pl line 960.
Updating Webmin user .. .. done
Saving server details .. .. done
Re-loading Webmin .. .. done
Well of course it fails to connect to localhost when I'm not running on localhost. :) I have the MySQL module configured to connect to the MySQL load balancer....hrm. Any way to tell it "no, ignore localhost, just do the rest"?