I just got off the phone with DSS, I was curious. The CM had to put me on hold to look for her info on the YC/BC cabanas, since they are so new. She found it, and there are the instructions to them that a guest renting one of the cabanas has to be a guest of the YC/BC and they need to confirm that. However, and this was news to me, she said that
any WDW resort guest can rent a cabana at the GF or CR! I didn't ask about the Patios at the Poly, she mentioned them as she was discussing how things are changing in that department so rules could be changing as well (meaning that it could be relatively new that any WDW guest can rent them). She said that she imagines that once they update the cabanas section to include the YC/BC there will probably be a new bullet point saying that you must be a YC/BC guest to rent one of those cabanas. I'm assuming that they won't be added to the website for quite some time, since Disney isn't known for keeping it very up to date.
It also just dawned on me that I should have asked her about the 6 guest maximum, and what if you had 5 kids, since it wouldn't really matter to me and then we would at least know the official response (from one CM at least!). Sorry