Persist based on query string

This sort of seems to be working:

         Direct traffic to that node.
        pool [LB::server pool] member [class lookup $webserver servername2ip_datagroup] $prt
        persist uie $webserver 518400

The first request for index.php goes to the correct node, whereas the resources (css, js, images) go to a random backend server. If I refresh the page though all requests seem to go to the correct server.

I don't suppose there's a way to make it work consistently for the very first request?