I would find it really useful if I could select an item and have a condensing change be directly applied to that staff at that rhythmic position, as opposed to having to open the dialog and then tick the appropriate condensing group. (And often having to do this a few times).
The dialog could still be available in order to customise the condensing.
In other words, this option would simply reset condensing from what Dorico would regard as the middle of a phrase without any additional customization?
Sounds like an idea. 
Exactly.
Perhaps it could tick all of the selected staves/groups if multiple are selected?
I agree that creating a condensing change can involve lot of clicking with the mouse.