There are several approaches and I'd encourage you to research them all so you have a very strong understanding of the water dynamics involved.
Typically the overflow method is used, as it's elegant and safe. Either a hang-on-back overflow utilizing a siphon (I personally distrust this type of overflow though they have excellent performance rates), or a drilled tank of some sort with a standpipe at the water line. The theory in either case is that water flowing down the overflows only does so because the return pump (in the sump/fuge) is pushing water up into the display, thus causing the water level to go up, and allowing it to enter the overflow. Make sense? The only reason this is the most common system is safety - what happens if the water goes out or the pump fails in some way? Water stops returning into the tank, and thus the water line drops in the display tank until it's flush with the overflow.. No more water can exit the display. Now if the power comes on again right away or in 20 years, the water from the display can't exit until more water is pumped in.
I've also seen a few rather impressive DIY solutions that were rather crazy but appeared to work. If you're an innovating kinda guy, by all means see if you can think of something that'll work nicely.
Your float switch and powerhead idea is actually pretty good, but there are two hitches:
1) float switches do fail. Eventually. Since the water capacity coming down will have to be capable of exceeding the water leaving, if that switch fails you will flood the house.
2) powerheads are TERRIBLE under a lot of head. Try pumping water up 2' and you already have a fraction of the original flow rate. For a fuge, mind you, it's not necessarily a bad thing.
I can't think of a way to draw a diagram that won't look terrible... What about hitting up your LFS and peeking at their plumbing?
