Our Head (prep school) was totally open with parents who went to see him on their child's ranking. Likewise a local state school Head was as well.
I would simply say to the Head that other schools are not saying this, so why is he? My guess is that he doesn't want you all rushing in to lobby him for better rankings! To prove that there is no such rule laid down by Bucks you can read the 11+ Transfer Manual for Heads here.
http://www.buckscc.gov.uk/schools/docum ... 1_plus.pdf
Note that it still relates to 2007 Admissions, and the 2008 Manual is only likely to appear sometime over the summer or early in the autumn term.
The deadline for the OoS last year was 17th November, so I see no reason why, on the date of this year's deadline and once the process is done and dusted, you can't ask the Head what your child's ranking is. After all, if your child is borderline for the 11+ it will at least tell you whether your would stand a chance at an Appeal (bearing in mind that the lowest likely ranking for a successful appeal is 2:1.) and give you a chance to weigh up your options before the result.