Thanks for all the advice.

I took MDtR's suggestion and split the stay into multiple reservations. Now we have one 5 night portion booked at the lowest rate, and the other 4 nights (with the forced upgrade) are half covered by our free night certificates. We're still slightly short, but it's a small enough difference that we can just buy the points if we have to. I'll check room availability periodically to see if anything opens up at the lower rate for the entirety of our stay.

I won't count on it the upgrade; it will just be a pleasant surprise if we get it.

Cheers!