Derailer cog capacity has to do with not just the derailer itself, but also the offset (drop) of the hanger from the axle centerline. To accommodate a couple more teeth, it might be possible to set up a rear end with a bit more offset, by moving the axle slightly. Tire clearance and chain length would limit your ability to do this, naturally. Of course, with a non-integral hanger, you could always change it for a longer one, if you can find such a thing.