Iron Man's not bulky, but it's gotta be bigger than a 3PO torso right? I mean, 3PO is one of the more slight characters of the entire saga.
Crack open a light-up R2 if you have one, check out the electronics. Better actualy to probably crack open the 3PO accessory Chewbacca had some years ago. That gives you an idea on the size of the electronics involved anyway.
I think the light pipe head ala that 3PO accessory is a good way to get the eyes to light up... the head basically being one big light pipe, and it's painted everywhere but its eyes.
I've not cracked a BAD protocol droid torso open to do a comparison of electronics and see if it would work.
Bearing in mind too, that torso would probably need reworked entirely, to accomodate anything it could perhaps fit into it. This would likely be an entirely new figure short of maybe re-using BAD limbs. I'd be ok with that, and it's maybe doable. That's just a really tight fit I think. I don't have one of those 3PO's available to pop apart or I'd give it a whirl with some extra BAD parts, just to see if it's even feasible.