Hope this story of our experience will help:
I've done two BCV add-on's, both thru Disney, both for 50 points. The first was July 2007 and the second was 3 weeks ago, in early December--a couple of days after we got home from our 2008 T-giving trip (I left Disney 12/2 and our guide called me a couple of days after I got home, in response to the message I'd left him while still at Disney)--We have a Feb UY.
The first time (July 2007) our guide told me only that he honestly didn't know how long it would take...but I gave him a range for the number of points I wanted/needed (40-50)...It took five weeks; we could have bought fewer than 50 (but I bought all 50)...I banked them immediately...but no "special" process was needed because I was well within my banking "rights" at the time time (this was before the change to 100% banking for the first 8 months and nothing thereafter)...
This last time, I told our guide I was buying for the sole purpose of making an 11-month reservation at BCV for T-giving of 2009, so I needed the points by 12/20 (or for him to use his "magic" to get us in at BCV if the points came thru after the 11-month window opened)...I needed 2008 points for the reservation, so I double-checked that I'd be allowed to bank newly-purchased points even tho I was past 2008 points banking deadlines...I did bank them, the same day I bought them...no problems at all doing that...it's standard procedure for newly-purchased points and MS knows it...I also told our guide that I wouldn't buy the 50 BCV points unless I could use them to make a T-giving 2009 reservation...
Our guide made no promises about his ability or inability to get us a reservation if the points came thru after the 11-month window...all he said was if it turned out to be necessary, he'd see what he could do...said to hold on and he'd check the waitlist...told me only 3 folks were ahead of me for Feb UY points...also told me that due to demand for BCV, Disney is "buying up all BCV points" so he was "hopeful" that my W/L would come thru "in time."
It did. I bought the points. I made my T-giving reservation on the first day of the 11-month window opening.
Hope this helps!!