doubtful
I don't think a panel would produce enough power to provide THAT much extra battery life, at least not from active use. If the phone were in airplane mode, then it might extend the standby time a lot.
Also, if people kept the phone out in the sun, it might receive a lot more juice would but, at minimum, the heat would be bad for the battery and worse could cause the phone to overheat and refuse to do anything until temp was low enough. Heck, it could get too hot so that the iPhone refuses to charge the batt anyway.