King of the Hill / Rewards for Area Control in Minecraft

If you have protected (uneditable) blocks, you can build this as a redstone mechanism.

Start with a long-period clock (there are several ways to do this), 5 stone pressure plates, and a dispenser. Construct the logic circuit not(or(edge-detector(clock), not(plate), not(plate), not(plate), not(plate), not(plate))) and connect it to the dispenser.

The dispenser will dispense whenever the clock triggers as long as someone is standing on every one of the pressure plates. (They can also be triggered by mobs, creating the possibility of alternate solutions such as one player with four wolves and a bit of patience to herd them.)

The dispenser will need refilling (the maximum capacity is 576), but perhaps you can find a mod for infinite dispensers. Or, if you can produce a device (such as a mob trap) which generates a stream of items of the type you want to dispense, then you can have the pressure plates wired to pistons to switch the item stream from an item destroyer such as cactus to the players.

Let me know if you'd like more engineering details.