You'd have to use AppleScript for that. That said, seems like the type of thing that Hazel is trying to avoid (i.e. taking the "automatic" part out of the equation).
I understand ... But I do not know how to use AppleScript. Anyway, I think it would be simple to add that Hazel. It was enough to put an option to prompt, as well as have the option to Replace File in the Options ...
If you have something else that might help, I'd be grateful.
I don't see it as something I'll add as it's something that goes against the problem Hazel is trying to solve. Maybe someone else here can come up with the script for you.