If not, why not?
I've seen very small, quiet gasoline generators. I would think this could solve this problem easily at the expense of a small amount of storage space.
Even if the generator was in a small trailer, that would provide the range freedom without loosing the storage space (and help with venting/cooling/noise).
This has been done; but the small trailer also adds significant friction and aerodynamic drag. It also adds complexity in driving and maintenance.
You don't even save much by keeping the gas tank mostly empty when you aren't expecting to need it -- the gas weighs ~6 lbs / gallon, so you might save 50 lbs by keeping the tank on 1/4 when you aren't planning to drive past the battery range.
Think about it this way: batteries are really heavy, and the Tesla has like 5x more battery capacity than a Volt. Meanwhile, if you're mostly making short trips, you're hauling around 3/4 of those heavy, heavy batteries as a contingency.
As others have said, it's more practical to have a purpose built hybrid.
From a regulatory standpoint, yes, fuel systems would be regulated in this design.