How about another option under Notifications for owner/mods that would send a summary of the previous 24 hours (since last summary) of the Activity Log?
I like this idea, if made optional...perhaps as a new checkbox in the existing Notifications section of a Mod/Owner subscription.
It seems like an expedient way to achieve most of the goals of the long-awaited "notifications overhaul" without attempting to create a separate notification for each and every group activity (which could quickly get out of hand).