If I do an accessory based for example on glasses and include the four mesh resources into the Bodyshop-exported package, would it be possible to clone it by just making a copy, renaming that and then doing a "fix integrity" on it?

Maybe... I'm not sure if it would change the stuff on the Body Shop file too - but I wouldn't recommend doing it that way even if it DOES work.

You CAN include the mesh bits with a recolour file and it'll work (a couple people were doing that with hair early in meshing) but I wouldn't recommend it - being in the same file, deleting the recolour, you delete the mesh, and I'm pretty sure doing that would also result in your mesh being uploaded with the recolour if someone tried to put it up on the Exchange... 99% of the time meshes aren't included but sometimes they are... and I suspect it's due to having the recolour and mesh together in the same file.

You're better off keeping your mesh bits separate (I keep a folder called Maxis originals with subfolders of each mesh name I've worked with with all the 4 bits inside each for easier cloning), making a new mesh package with those, fixing integrity, exporting the new CRES and SHPE and then attaching those to a fresh recolour package from Body Shop.