Hi Chris,
That said, you can easily add it (or any missing file format you find) to your site by logging in as an Admin and going to "Registries" -> "Formats". From that page you can add new formats as needed.
By default, DSpace only comes with a small set of pre-configured formats. That said, as you noticed it will **accept** any file you give it. But, if it doesn't know the file format yet, it just says it's "Unknown".
(I cannot answer your question about whether adding this JP2 format will cause Cantaloupe to work with it. It might, but I admit I'm still not an expert on Cantaloupe & the new IIIF features -- so, perhaps someone else on this list will know the answer.)
Tim