MyBB Community Forums

is this doable via promotion system or something?

It is possible with a plugin. A developer can hook into the task_promotions hook (/inc/tasks/promotions.php) and use some additional logic to figure out which UIDs will be upgraded and send a PM accordingly. If an additional hook were present then the task would be easier if you could access the raw UIDs that need to be promoted without extra code logic. Patches plugin would come in handy here if you use that. With further manipulation a developer could add database table column(s) to the promotions table and hook into the Group Promotions AdminCP page and allow you to set your desired PM all from the Group Promotions page.

So yes, it is possible with a single plugin file.

Sounds complicated, this should be a core feature as it has lots of usage for forums which have lots of groups and those which gets promoted via task system.

I would been interested too