I think you have spent as much time today thinking about this as me
Your idea about wood under the lid, yes, I think it could become disastrous. You can fit a suckling pig on a large smoker, if smoke is really important, but the cajun mic is meant to be a huge oven rather than a smoker. However, with that said....
I have seen a few that use a ground level fire and the pig above it, I plan on building mine with no bottom so as to provide options for this (and it's cheaper). I reckon that, depending on the construction and size, you could put a small indirect fire in one end and add wood to that if desired. Of course, this would require ventilation, but that's not hard to include with a movable baffle to close it off if only using the top coals.
WRT the wood burning, I don't think stainless steel is going to provide much more protection than tightly wrapped foil due to its conductive nature (I could be wrong on this). Most designs that use wood (as opposed to say, breeze blocks or bricks) have a fairly wide border around the fire bowl. Does anyone reading this have any idea of the rough average temperature inside the beasts? Also, if the plywood is really tightly wrapped, with foil tape on the seams, could smoke actually be generated and escape?
As for the cost of foil, I don't intend this prototype to do lots of cooks. It's not for commercial use, and I imagine will mostly be for doing large roast dinners when I have some extra family round. I can't afford to burn that much charcoal on a regular basis - though waste wood can be used too of course, as the fumes will not be in contact with the meat. As such, and given the experimental nature of this, I'm just gonna stick to foil for now. If it catches fire, so be it
Pig cost - to give you an idea, I can buy locally raised, free range boar / mangalitsa cross pigs for around £280. They tend to weigh in around 80lb (roughly). They come whole, but cleaned, and I have a friendly butcher who will joint it for free. A quick look at prices on the web show between £200 and £350 for a 100lb pig, depending on how much butchery you want doing (those top end prices include bacon, sausages etc, but I would rather make my own).