I actually kinda DON'T want this warning. At least if it was implemented, I'd like an option to shut it off.
Why? Think about how often it would be going off. Any time you went to channel an enemy tower without minions, and constantly during big fights on top of towers. Since the best way to fight on an enemy tower is to alternate brief channels among your team, it would ping about every other second. Eaugh.
I also like having the responsibility of knowing for myself what a tower is doing. Sometimes I don't know it's target has changed to me and I get pounded, but I accept that was entirely my fault. Sometimes an enemy thinks they have an easy backcap and has their guard down when you interrupt them. Sometimes they respond fast, but sometimes they're paying so little attention to themselves it takes a moment or two and they eat some lasers before they wise up.
I like that. I think tracking tower targets is integral to Dominion and I don't want that to get easier.
Dominion towers are less lethal than SR towers anyhow, so we already have a wider margin for error there.