See... It's kind of a quirky thing. There is a trend in the modder's section about adding chargers to mods so they can do what you are talking about. Maybe some of the mods offered commercially have them for 5 volts.... IDK. You may wish to check the vv mods too.
I comment because many mods have REMOVABLE
batteries. Thus, people use external chargers with them and just swap out the
batteries (10 seconds) and then charge (with high powered external chargers). They still don't lose any
vaping time that way, and don't have a cord hanging off the PV.
It's definitely a thing with those mods that have fixed non-removable batteries since they have to be charged somehow, and they sometimes build the rest of the charger into them and use the mini-usb connection to charge em. No downtime that way. How many have removable batteries, 5V AND a charger... IDK. More likely to find a match with a non-replaceable battery. That's what's so cool about the mod market tho... variety! I think you'll find some. We kicked it around here some:
http://www.e-cigarette-forum.com/forum/modders-forum/195659-someone-explain-me.html
I (and I assume you) would love to see them come out with a 5 volt eGo style unit, even at net 600 mah per dual cell "battery". PT even better. Charging dual cells internally is trickier. Booster mods are generally larger, and more expensive, but would be easier to put a charger into. Most are vv, I would think. (This is all a bit of ramble/speculation on my part, as I am in no way an expert in all mods on the market).