I like your approach to the challenge. Having a clear concept is the way to go and sketching out that idea nails it down even more. Otherwise, you risk ending up with something that is disjointed or something that looks nothing like you wanted. It can also slow you down your progress quite a bit. A little bit of planning up front can save you alot of time at the end.

Part of the judging criteria is creativity so I think your latest idea would work really well. Anything outside the box usually reflects the artists creativity so I say go for it. Ask yourself "what can I do that will make people think...WOW!"