Although, beautiful and highly recommended, Unknown Pond is kind of out of the way if your looking to only hike Cabot. You'd have to hike over Horn, then Bulge, and THEN Cabot coming from that direction. Horn definitely makes it worth while if you're willing to take on the extra mileage and elevation. And at Unknown they aren't platforms, but hardened earth sites.
Depending on how many tents you have, I'd chance it and find a flat, legal spot a little ways down the old *Cabot trail or off Bunnell Notch to base camp at.
*That's a little ways down from Bunnell Notch, where the old Cabot trail is still in the WMNF and, hence, legal to camp. Accessed via Bunnell Notch Tr.