Andersson has made a backpack addon.

As Andersson says it is far from perfect but it works. We hope to help him clean up the scripts and use some of our spoofing tricks here at the CoC so it no longer requires an entry in the init field and the crate will no longer have to be hidden underground.

At Andersson's request this version of the addon is open source any one who want use it to develop from it can.

Description by Andersson 

Its a backpack-addon (secondary weapon) to which you "assign" a special crate (cargo capacity 4 weapons, 20 magazines) and when you drop the backpack the assigned crate are moved to the units position and 1 meter under ground so you cant see it. But you can take and put weapons to the crate (name "backpack") as you would to the backpack. When a unit picks up the backpack the crate goes to pos [1,1,-10].

There are issues and basic scripting and so on but it works for me. So if you need a backpack at all cost then its maybe ok? Or if you want to make something of it you are welcome.