I do know the Alta 12 vdc circuits are sensitive to loads. I had to replace the tail light bulb that came with the bike, because the bike didn't like how the bulb "felt" and shut the circuit off. Bulb was good, worked fine if I started the bike with the head light in high beam setting, but as soon as you switched to low beam, it would shut the tail light and license plate LED off. As soon as I replaced the bulb, everything was fine, so there is some sort of load sensing circuit and software code in the Alta's and it can shut things off if It doesn't like what it sees.
Normally, a USB charger draws 500-900ma so less than 1 amp (6-12W). It isn't much, but I was told by Alta that the EX couldn't charge a phone so I haven't tried it. If you can trade off loads you may be able to make it happen though.