I'm using Fancybox.

You can see the code at http://www.brettkaufman.com/staging/art.html.

I cycle either through the Assemblages gallery or the Archival gallery.

How do I make it cycle through the entire combined gallery, and not just the subsets?

Well, I figured out my own inelegant solution. When the page opens, I have jQuery set the REL attribute of all the thumbs to "todos." I put the code on all of the buttons as well. The "Assemblages" button finds all of the elements with the class "assemblage" and sets the REL attribute to "assemblage." I'm sure there's a better way, but this gets the job done.