Finally found it!
Here's the original thread:
The answer is that the child would stay on the waiting list if at a different grammar school in Bucks.
It does raise an interesting question, however - there seems to be a "grey area" in which anyone wishing to apply after their qualification lapses is effectively excluded. Apparently you need to have "exceptional reasons" for wanting to be tested outside the normal round - so, presumably, simply wanting to stay on the waiting list for your preferred school isn't enough. Wouldn't it be fairer to make the qualification valid until the following April, which would be the earliest opportunity to sit the 12+?