I'm in mo and I would be comfortable splitting a hive with a box full of drawn comb, stores, etc as late as drones were flying. And that only as long as I was letting them raise their own queen( actually drones would have to be flying 30 days after you split ) If you were buying a queen I wouldn't know why you couldn't split anytime you can buy a queen and the bees haven't went into their winter cluster.
I'll more than likely start my last nuc of the year before the first of August. I'll do it with 6 frames of bees, let them raise their own queen, feed the tar out of them and have them in a full size box by winter.
If all goes well, if not I'm not out anything.