If that's what you're thinking, I'd go with lightweight mats (2ply front, 2ply back) so they fold up easier. You'll need to make bookbinder's type hinges between the mats so it will accordion fold. I suggest experimenting with some Manila paper for proof-of-concept. I've actually seen people do something like this before, and the biggest hassle in the construction is the planning and prototyping. It might be better to do this not as overmats at all, but just drymount with very careful positioning of your images and labels, and even more careful positioning of your hinging tape. If you can find a very long piece of manila paper that can be accordion-folded, you can use that instead of tape hinges, and it will look more elegant.