You can go a couple ways, I think.
Try asking EdT, who might be off racing through the high country at the moment if he can recommend something. He's familar with alot of gear.
You can look for such a pack yourself. Try a web search. Just because some guy at REI can't help you doesn't mean it can't be done.
If you really need to carry much in the way of a load, you might see if you can get a custom built item from one of the more prominent makers. Doesn't take much to send them an e-mail and ask. E
