I've been thinking along these lines as well. I've had some bad moments when I did not know where my chickens were, You could put RFID chips into legbands and get a USB reader to write to a Raspberry Pi.
Now RFID is only going to scan them when they walk past it. But the Raspberry Pi's are cheap enough that I was thinking of putting one on the coop to the yard door, and one on their ususal path to beg for goodies. I think I can run them off a small solar panel and a battery like the PulletShut door guys do. Maybe one on the other side of the house, as well or at the bottom of our property. Not sure. Raspberry Pis are tiny and dont need much power. The programming would be simple enough, just log the "detect chicken" event, chicken ID, and time (location is impled byt the Pi that logged it. Then send that to an email addy or a website. You would then know when each of your flock passed a certain location and what time.
It would at least give me some sense of where they were when last sighted.
I'm giong to work on this and I'll post about cost and complexity.
I love it when chickens cross technology!
Leslie