Part of the problem may be that the USB spec is not designed for high-wattage output. Phone chargers can go higher, as you pointed out, but that's a pretty specialized situation. So there is not much incentive for a manufacturer to design a high-output LiPo charger that runs off of USB. Meanwhile, 12 volt batteries are cheap as chips and common. Not the best for backpacking, but there you go.
If you are determined to go with a USB system, look into buying a boost converter
like this one. Create a cable that has a USB plug on one side and a DC barrel plug on the other side, with the boost converter in the middle. Bob's your uncle.
Do be sure not to over-draw your system. I'm not sure what the effect on current is when doing a boost conversion, but I'm willing to take a stab at it. The boost converter must draw increased current on the input side to get the higher voltage on the output side. The energy has to come from somewhere, right? If I had to guess, I would say the right way to think about it is to use power, which combines the effect of both current and voltage. If you have a 12 watt iPad charger, then 12 watts = 1 amp @ 12 volts. So I would guess that the max you could pull on the output side would be 1 amp, and that doesn't account for conversion losses. Subtract another 10% for conversion losses and you get about 0.9 amps max current available. Since the charger itself is not 100% efficient, the realistic charging rate you might expect to achieve could be something like 0.8 amps.
