I don't see why you couldn't use any bladder in any backpack, as long as it physically fits, and the access for the tube is in the right place.

BTW I tried mine out skiing again today, -26C, windchill -36C, and it was OK as long as everything was under my outer fleece layer, including the mouthpiece. The only thing is I look like the hunchback of Notre Dame with the backpack under my jacket!

I found the neoprene insulation for the tube at the MEC store last week and it was $16.00 Cdn. I think I'll look in the plumbing supply and see if something there will fit!