After thinking about it some more, we decided we might look at not using the 1/4" and just re-using the 1/2" PT plywood. It wasn't too bouncy at all and that was with simple 2"x2" (1.5x1.5 finished) cedar. With the aluminum, I expect the frame itself won't bow/flex at all (not detectable, anyhow).
I'd have the long rear edge sitting on the bench (100% supported along that rear section), and then 3 aluminum frame rails about 2.5' in length running forward/back, and meeting at a 90* angle, the front 4.5' frame rail, which would have two legs on it. That means the platform is roughly divided into two sections, each with a frame around it. Each section would be about 2.5' x 2' in size between its surrounding frame. I can't see a 1/2" plywood section of that size flexing much according "The Sagulator" here http://www.woodbin.com/calcs/sagulator.htm#edge_strip
I know that's for shelving, but using a 30" square "shelf" to represent one half of the platform, and assuming the frame we build properly supports it, using pine as the shelf material, and a 2x2 "oak" front strip (to represent the front portion of the frame), the deflection with 400 lbs of load, centre-weighted, is only 0.01".