I've gone a number of different routes having to do with topping up a carboy during aging - marbles (can't trust them and they're a pain in the butt), different size carboys (there's just to much difference between 23l and 19l (6 and 5 gal?)), carboys and gallon jugs or 1.5l bottles (again a pain as it's difficult to clear and sulphite both), topping up with store bought wine (just doesn't seem right) and probably others.
I've decided to think about somehow leaking CO2 right over top of the finished wine while in the carboy. Perhaps a 2-hole stopper for the air lock and a pipe that can extend down to the top of the wine. Connect a cannister and slowly leak CO2 into the pipe letting most of the O2 bubble out the air lock. It wouldn't be quite perfect but done slowly the heavier CO2 should displace most of the O2. Use the bubbles out the airlock for metering and then close off a valve and disconnect.
Any thoughts about the technique or where to get the hardware?
Don