It's heavy, but it will work. I think you could use 1/8" aluminum flat stock which is enormously strong, pretty cheap, and 1/3rd the weight.

I do think you can do the wood thing with regular panniers if you want. You bond the ply together, and across the top. Where you want to hang the panniers just bond some 1/2 round along the edge that will support the hooks on your panniers. Let the hooks penetrate the ply. In the areas where the hook will penetrate, remove the wood and replace with solid epoxy so that where the hooks go through the ply the hole is in the plastic. By the way, the ply is fiberglassed. So it is pretty bulletproof. In fact the half round could be a little blob of epoxy putty.