Incorporate Canister with Sump?

Good luck with this. It's a great idea in theory. I've talked with a lot of freshwater cichlid folks, and I myself have a freshwater tank with a sump. Sooner or later somone gets the idea to try what you are planning. Getting the valve throughput balanced with the pump output is an almost impossible task. Even if it is off by 1 ml. every 10 seconds, which would be undectable to your eye, that's 360 ml per hour. In a little over 2 hours you would be out of balnce by a full liter of water. You can see what a challenge this is.