Can HAProxy select a backend based on a lookup table?

You can use maps to make it work.

This is perfectly outlined in a post on the HAProxy.com blog. They're mapping based on the Host header, but it would be trivial to change the config to work for the URL.

One extra benefit here is that you can dynamically add and remove mapping entries using the http-[request|response] [set-map|del-map] keywords, or via the admin socket.
Changes made with http-[request|response]or via the socket aren't persisted between reboots, so you'd want to have some out of band process to update the map file at the same time, but that's another question all together.

Assuming you have a backend for each server, and given a map file called /etc/haproxy/items.map with these contents:

#itemPath    backendname
/item/a      bk_server01
/item/b      bk_server02
/item/c      bk_server02

You would do something like this in your frontend:

frontend ft_items
  [...]
  use_backend %[path,lower,map(/etc/haproxy/items.map,bk_default)]

If you have both servers in the same backend, you'd write an ACL in the front-end that catches all the items (acl items path_beg /items), send them to the backend (use_backend servers if items) and replace the use_backend line from the frontend with appropriately if-claused use_server lines in the backend.

Yes, you can easily do this with HAProxy. Simply create an ACL with a list of IDs served by server and apply it to a use-server directive. IE:

acl server1-ids path_beg -i /items/id1 /istems/id2
use-server server1 if server1-ids