While this would be pretty involved, I was considering putting everything in the Azure Data Catalog.
From there, I'd create a publicly accessible web page that hosts end-user-relevant definitions with a GUI that facilitates easy searching.
The web page would use APIs to access the Azure data and automatically reflect changes we make to the Catalog.
On the BI front end, there would be a standard object containing a hyperlink to that web page so that the user can access the information easily.
Obviously, not as good as automatic integration with hover capability, but it would be a start.
I suppose you could actually wire a visual to show definitions of any measures used on that report, too, but you'd be using screen real-estate for that.