I may be way off the bat on this one, but here this is my suggestion.
Why not have the assembly diagrams open in a popup window and have them request the individual product info be displayed in the store/main window. Add a close button and maybe a right/left arrow navigation scheme for browsing categories on the pop up window. I think you could hardcode categories.tpl to include the open window behaviour so that it would stay dynamic.
Would "_parent" work for calling back to the cart? I dunno.
My idea may be too simple. I'd be interested to see your solution to this.
