iPad adjusts brightness when it reaches full charge
First, to directly answer your question, no, there is no hidden setting that is causing your brightness to increase without your input. The only to settings related to brightness as of iOS 6 is Auto-Brightness (which you have disabled), and the brightness slider. As far as I'm concerned, it must be a bug.

So it looks like you have a few sub-par options. 


USB outlets are really nice, but in the long run, they may not be a good solution for you. Even though the original power supply bricks that come with iPad, stick out of the wall, and can be in the way, they do supply enough energy to keep the iPad charged, and even charging despite high battery consumption at the same time. 
Bring one home to see if you can replicate the problem with original charging apparatus. If this is the case, you may consider scheduling an appointment, and taking it to one of Apple's Genius bars. Perhaps they are aware of a bug or will have an idea as to what is going on. 


One side note, I have seen extremely strange behavior when an iOS device is given "bad" power. If the USB outlets are questionable quality, there is a far-out chance this is causing it. That said, most of the behavior I have seen from "bad" power comes from touch screen activity, and this would mean that each device was used in between times that you checked on it.

Either way, my recommendation would be to use the included chargers with your iPad if that is at all possible.