By Yum 0.8, these are the table variables you can set (taken from UserModule.php):
public $usersTable = 'users';
public $settingsTable = 'yumsettings'
public $textSettingsTable = 'yumtextsettings';
public $messagesTable = 'messages';
public $usergroupTable = 'usergroup';
public $userUsergroupTable = 'user_has_usergroup';
public $profileFieldsTable = 'profile_fields';
public $profileFieldsGroupTable = 'profile_fields_group';
public $profileTable = 'profiles';
public $profileVisitTable = 'profile_visit';
public $rolesTable = 'roles';
public $friendshipTable = 'friendship';
public $permissionTable = 'permission';
public $actionTable = 'action';
public $userRoleTable = 'user_has_role';
public $userUserTable = 'user_has_user';
public $roleRoleTable = 'role_has_role';
public $activityTable = 'activities';
This is a quick & dirty fix. I don’t like it, but it works.