Aha.The pitch in the sports car press here, years ago, was that the plywood chassis required too much labor. I don't have any other information on it, except that the same comment was made in one of my coffee-table car books. The steel space frame was cheaper to build at the time. I'll bet they brazed it, like a lot of specialty Brit car manufacturers did.
Plywood actually is a very good material for making high-performance chassis. It's good for shear panels and it's good for those multi-box tubs, the early ones, that were sometimes called "monocoque" but which weren't. They were a collection of shear-panel boxes, generally made of sheet aluminum, but they could be much stiffer in plywood. They've been used in a variety of structures but I don't know of any cars built that way. I've heard that the "appropriate technology" mini cars that Renault had designed for indiginous construction in Africa a couple of decades ago were designed that way, but I never saw one, or even photos of them.
The Marcos, IIRC, was a little different. I saw some photos of the old Marcos wooden chassis once upon a time, and it looked like the frame of a big hydroplane or plywood airplane -- circa 1960. It was a highly engineered structure. My recollection is that it was a series of crosswise bulkheads tied together with upright longitudinal panels. It looked like the skin might be stressed, too, but I don't know.
You probably know this but it's a potentially useful point worth repeating: a good-quality plywood, like Bruynzeel or one of its copy-cats, has a weight/stiffness value, and a weight/strength value, roughly equivalent to a medium-strength cored composite. For example, S-glass fabric and epoxy sandwiching high-density polyurethane foam. Twenty-five years ago, that was a high-performance composite.
The performance of a well-engineered plywood structure of that type is at least as good as, and maybe better than, a fully triangulated tubular spaceframe.
I hope someone got that down on paper or other recording. Too many of those gems just get lost.