Welcome to the forum, Chris. My gut feeling is that if BGT was accepted by Pre '66 FIA regs we would see them racing alongside the hard top cars. We don't.
My understanding is that the defining date is when car officially went on sale, not when made. It seems from what we read that this was 1966. It is quite possible dealers registered cars during January, if they had received the necessary paperwork from MG to do so, even if that was pre 'release'.
