Do you think that a "plate" of 1/2" plywood will provide enough holding power for the screws? I'm not planning on mounting anything heavy except for a heater, which I might mount on a shelf. If not, I see the wisdom of your plan.
Can I ask how you create such nice illustrations? I studied drafting and design for a while and even worked as a draftsman during my college years, but I'm stuck with paper and pencil, I've tried sketch up and another free autocad drafting program without much success (probably a lack of patience). I'd sure like come up to speed.