For charging airsoft batteries? Very unlikely, as you'd have to step 5V up to something like 13V to charge an 11.1V (and actually peaking at 12.6V) lipo, and you'd be charging it at under 1A.
What problem are you trying to solve? As in, what batteries do you already have? The default recommendation is a SkyRC S65 as a do-anything solution (caveat: unless there's something better and cheaper out there now).
Er, buy another one? Unless you're emptying drum mag after drum mage, 2.9Ah should see you through a day - I usually swap batteries at lunch to spread the load.
Failing that, a 12V -> 240V inverter should be fine for running a mains charger (assuming you're not a bus wanker). The older SkyRC iMax B6 chargers will run directly off of a 12V (well, 12.5V to 14.5V) car battery - I think mine came with a crocodile-clipped lead for that purpose. That's not a recommendation, those are old tech now, just a note.
But they take a good while to charge. They can get you out of a hole if your site doesn't offer battery charging. Frankly though as said - it's easier to buy a couple of extra batteries.
Oh, nice, but as you say, a last resort if you want to put a little charge into a battery on site. Might make sense if you're at a weekender with no charging facilities, and don't mind risking your batteries to whatever circuitry is (or isn't) in there.