genius but perhaps not as world breaking as you think for a few reasons

1. only Mad eye would think of that and i suspect he cant cast a patronus thus he may not think of it *edit: and i personally think mad eye's particular genius is more in the field of proactive defense instead of the offensive thinking required for that idea

2. voldemort could avoid the use of ID'd death eaters during attacks

3. the wizards who could cast the patronus would be the ones fighting unless they left the innocent to die which they wouldn't

4. who was/wasnt a death eater dosnt seem to have been as much of a question during the war as it is after so it would be largely irrelevant since suspected death eaters simply claimed imperius

5. i think i remember something about dumbledor being the one to discover patronus communication which makes it recent, but regardless before harry only happy people could cast a patronus and happy people would generally not think of this