You can find this addon over in Addons:Beta Testing...it should be one of the first posts on the page, then down towards the bottom of the thread there's a freewebs download link.
So to address your points:
1. No, I don't think so. AFAIK the bag has to be dropped (more realistic I think, you drop it and then open it up). So unless someone needs it, the guy is just running around with this big bag on his back...it's coded like a secondary weapon.
2. If you're the squad leader, there are no problems. you can use the radio action menu to take care of that easily. You can add the rucksack as a weapon to any guy you want, as long as he doesn't have an AT launcher on his back. Again, it fits in the secondary slot, so you can just add it in the init field of any loony you like.
I was thinking of a rearm script for you to use if you weren't the leader, but had decided not to push my luck too early with that idea
We know that your character will automatically report "AMMO LOW" to your leader - you could use that event to have the group leader then initiate the AI rearm script (if someone thought one up based on my first post). Your leader then tells your squad's ammo guy to drop the bag, and then you can go up to it and take what you need.
There's also some event that changes your little soldier icon from red (when you have low ammo) back to normal. Maybe that could be used to signal the ammo guy to pick his bag back up, since he won't otherwise.
So in general here's how it works: you can add the rucksack to any guy you want as a secondary weapon (just not an AT guy, since he already has a secondary). If you're the group leader, it's easy to make people rearm. The problem is getting the AI to do it and also getting your leader to do it if you are playing as just a squaddie.
And I should note that there is, for some reason to do with the scripting, about a 15 second delay before the bag becomes "active" when you drop it. In other words you have to wait roughly that long after it's been dropped for anyone to be able to take anything from it. It would be nice if someone could take a look at the coding for it (andy said it was messy) and see if there's a way to fix that. It could be a hassle to wait that long in the middle of a firefight.