Derek,

I was able to accomplish this using Assignment rules for one particular Catalog Item and it does route to the appropriate Assignment group for the location of the user.   The assignment rule runs on the sc_task table and the conditions are built specifically so they only run for this one catalog item.  

If you are still looking for a solution, here are the conditions I have used so you'll have an idea.   In my case, the user's locations are also populated from AD so at the end I do have a "catchall" rule in case the locations do not match any of the states/cities/countries I have set up.

Hope this help you or someone else perhaps!

Cheers!

Chandra