This is a great idea, thanks for posting, and please keep us updated on how well it works.
For those of you wanting a cheap autofeeder, I have 2 of the 6 gallon feeders from moultrie, and they work great, cost 25 each on sale. Got them through gander mountain on line. They run on a battery, but that last 1-2 months, for 5 or 6 dollars. They hold almost 3/4 of a 25lb bag, you can adjust how much it throws, and there is slight ajustment on time of day, usually 30 min before and or after sunrise and sunset. The only issue we had was a squirrel at the internal funnel, spilling everything out, but they are cheap to replace the funnel.
Just a thought, not trying to highjack the thread. Its just a super cheap way to get an automatic feeder, save some money on corn, but still have the desired effect. I may use your plunger method along with the other feeders.
"If Darwin saw the same sunrise and sunset, that I see from a duck marsh, or my tree stand; then he would know there is a God, and that He created all of it." -Ford-