I assume your overflow is at the top of the tank, you could try injecting food into the tank towards the bottom of the tank with a turkey baster. It would take a little while for the food to float up and sucked into the overflow. This is of course you feed with food that sinks, like pellets :-)
Second, ya, reducing the amount of siphon into the sump is probably a good idea. This is what I do.. a couple of holes drilled in the back of my return outlets (just below the normal operating water line; important because if it is above the water line you'll inject air in).
I personally do not turn off my return while feeding. Just my two main powerheads. My fish are quite voracious eaters and very little gets into the overflow by the time they are done eating. Instead of dumping all their food in at once I feed in two small intervals though. Dump in half their food, wait a little bit(let them polish it off mostly), dump in second half of their portion. I suppose that might be a pain for a tank sitter though.