It can be done. We got in this year for June 8-10th with a 5 1/2 month advance reservation (1 bedroom/3 nights). The key I think was lots of flexibility on dates, number of nights, and room type.
I have basically the same question. I need a studio for June 30th to July 5th and I see a very good deal on flights right now but can't purchase them without knowing if I can get the studio room for those dates. How fast does it fill up during the semi-busy season?