[Andrea Rennick]

DUDE, that was it exactly – I was missing this part:

// You just told bbPress to use that mu database to look for users, but it doesn’t yet know the name of the *table* to use.

// Change ‘wp_users’ and ‘wp_usermeta’ to be the table names of mu’s user tables.

MU also has a wp_users table, which so totally didn’t work when you point it there. Will try the above and report back.

(the plugin didn’t help btw, at least not originally)