Update as promised:
So Bill from Northwest Vapors (the one Vapor eyes er linked to above) got back to me last night. He told me the charged up passthrough battery pulls 4.2v unloaded while disconnected. He offered to give it a whirl for the evening and here was his response.
"The pass-through with the battery is a nice feature. The only thing to keep in mind is that the battery will last longer if you discharge it before you recharge it. You know, let the battery run closer to dead before you plug it back in. But that's not practical with a pass through. So generally a pass-through battery will not have the lifetime that a regular battery does because of the increase of the charge cycles on the pass through battery".
Side note ... My pass through is now registering 3.98v with no load after an evening of use."
So for those who are at a computer, this will be a good option for a backup battery for the KGO. Of course, unplugged it will eventually dwindle down below the 3.7 benchmark we all like.
But plugged in, it seems to stay at high voltage
I really can't say enough good things about Bill. Everything we are looking for in these Mom and Pop vapor shops and more.
KGO and eGo USB Pass-Through with 1000mAh Battery - $22.95