If made of plastic in a concertina form it won't weigh too much,
Collapsable knapsacks already exist, but the concept of filling them with whatever liquid is on hand is novel…
The pump is built into the wand, sorry to hijack the thread Jack, but does this idea have some merit?
Other models
http://www.bigredtruck.com.au/media/...5/4/0/4002.jpg
Vallfirest # Ergonomic Fire Fighting Back Pack - BRT Fire and Rescue Supplies