Well, I'm not yet very educated on the matter,

but there are tri's out there of similar design that will fly 20-30min with FPV gear and a couple of batteries.
900g is still quite a lot for a tri that size, I can't really see more than 10min with an efficient tricopter (undermounted contra-rotating props, lightweight frame and large props...). I'll get a better idea after further experimenting.
However you could have quick swappable batteries, you'd be limited by ammo anyway...
Or just mount a few paintball grenade shells and go for a scatter effect !
traditional fixed wing UAVs are pretty cool too
Or something using a free wing tilt design maybe...
The arms I had on this weekend are 60cm long, prop shaft to CG, but that's mainly for stability reasons when using a basic Tx and also as a consequence the arms tend to get shorter after a few crashes.
I've flown it with 30cm arms also, and they can also be made foldable.
So, yeah, fitting it in a back pack should be easy.
Although probably not feasible for small copters, a hybrid power system (IC generator powering brushless motors) could be interesting. It seems even low efficiencies would provide more energy density than current battery technology.