Nice . . . . I would've never thought to do that. I can see some other applications for that as well if you didn't have to worry about the noise.
Very clever, but it's a lot of wrenching just to charge a battery. On many vehicles it is not a simple process. Then you have to put it all back together when you're done. I would think anyone resourceful enough to carry a chain saw, a fan belt, and a set of tools around with them would also have enough forethought to bring along a $40 Harbor Freight jump box and avoid having to go through the hassle in the first place. But yes, it is a cool concept
OR, one could build a simple rig, similar to a ChainSaw Winch, and turn an JunkYard Alternator, with the ChainSaw Engine..... Just think'en....