Plywood alder does not hold light stains like solid alder (knotty or clear). Plywood alder/knotty alder will stain with a much redder hue because of the plywood glue. Keep that in mind if you plan on lacquering or sealing with a clear poly.
Other than that, I think the overall design would mitigate my approach to using ply or not.
-Lud
As Peter recommended, I'd use shop sawn veneer over BB plywood. Looks like solid wood, because it is, with none of the drawbacks. And because it's made from the same wood as the rest of the project, it will take stain and finish the same way. I also agree with others that said a panel that large will look incredibly boring. I'd show the client some alternative sketches, if nothing more than to have her recommit to that big panel.
John