Hi Alex.
I'm the product owner for Perform.

What I read from your post is that the client requirement is to allow managers to assign activities to individuals on an adhoc basis?

If this is correct, then unfortunately the answer is no.  Assignment is an admin only function, and assignment to individuals is not possible in v13. 

A possible solution is to assign all activities to all users and have the user self select the relevant activities with reference to the manager.  But that might not suit your particular requirements.

We are in the discovery phase for v14.  If you wish I can raise an improvement request so we can gather feedback on the popularity of this feature from the partner community.

Thanks