So anyone in the main mod usergroup will be the main mod of every forum they are set to mod?
Suggestion: Add a field to the moderators table (not sure of the name) for the main mod capability and edit forum.php a little bit so that it will add a yes/no option for that user to be a main mod of that forum. That way, Mod A can be a head mod of forum A, but a regular mod of forum B.