That'll be something to consider in the future. The notification system needs a complete rewrite to be honest. It's just not reliable enough to know who to notify.
For now you can try making a copy of the comment chain notification and adjust the custom PHP based user parsing to try and include tagged users. This should probably just be its own system action though and have added a feature ticket to review implementing this.
forge.joomlapolis.com/issues/8418
In that rewrite the notification system needs to become a subscriber system. So for example if someone is tagged in an activity post they are then subscribed to that activity posts notifications and if they've comment notifications enabled they'll be notified of comments on that activity post. This gives us a guaranteed list of everyone who needs to be notified. I do not have a timeframe for when that will happen, but likely not until after CB 3.x where we'll be converting CB plugins into standalone Joomla extensions.