casting bar from non targeted mob

[Bordoh]

I'm not sure if its possible, but is there a way to have a casting bar show from a mob you are not targeting?

The reason I'd like this is because my assassin has an instant cast ae immunity that puts me in stealth but only works while in stealth. When I'm on adds for a fight, it would be very useful to know when the names ae is going to go off, so i can go immune just before and cancel it fast.

Thanks!