Self tagging will eventually be allowed in CB Gallery to tag people in the photos (including self), but beyond that no other self tagging is planned. They can mention themselves in CB Activity though using @username usage, which is how you'd do it on Facebook and other social media.

I understand what you're using it for, but that's not what it's designed for. The tagging system is to tag who you are with. Posting that you're with yourself doesn't make any sense to the context of CB Activity. I suggest using @mention for your usage instead.

There's already a notification system action for tags. Specifically "CB Activity - Activity Tag Notification". If you mean mentions then there is no such notification at this time. Please keep in mind tags are not mentions. They are 2 completely different behaviors. You can only tag your connections. You can mention anyone you want.

As for trigger behavior on mentions there are plans for that in a future release in addition to filtering behavior (to filter down to all posts a user was mentioned in for example), but that won't be until after the storage rewrites in CB Activity as I need to implement additional database tables to keep references of mentioned activity otherwise it'd just kill the database trying to query for it. You can see this issue with hashtags since those don't store in the database and just do filtering on the query that isn't very efficient. Those too will be stored in a future release.