I would suggest booking your missing night on cash. That way, you at least have your reservation. If you can get it at 7 months using the one-time use points, then you can cancel your cash night. If you book through MS, they can link the reservation so it is treated as one, single stay.
We are going to *try* to do this for our HHI trip next year. We usually book a 1BR but we invited my parents to join us so we'll need a 2BR. Since HHI is an "every-third-year" trip for us, we used all of our banked, current and borrowed points to secure 4 of the 5 nights. I booked the extra night on member cash (currently only $250, but that is the 2012 price) so that we have all 5 of our nights. In order to book all 5 nights on points, we are 7 points short. At 7 months, I'm hoping to buy 4 one-time use points and combine them with my 3 straggling AKV points to book that last night. If I am successful, I will cancel my cash night.
(We actually have 2 nights booked on cash right now. We were planning to arrive on Easter Sunday, but decided to come in on Saturday night instead so that we can enjoy Easter morning activities.)