Ah, I misread your post.
Regarding category select, you can do standard calibre searches in the mobile server view.
I assume you are building all the links yourself, or generating them somehow, for each selection criteria -- not using the actual server page?
You just need to pass a parameter to
http://127.0.0.1:8080/mobile?search=%s for a search that would fit into the search bar in the calibre GUI.