It seems that leaks tend to have the opposite effect, weakening the fundamental and causing octave splitting/jumps in the bell notes, rather than making the first harmonic less accessible.

One thing about Low A horns is that the taper of the bore beyond the bow is sometimes different than the taper on the body.  It's a design compromise to keep the bell a reasonable size with the extended bore.  If the bell section has a small enough taper, or in the case of the Conn low A simply a cylindrical extension between Bb and A, you may get overblowing characteristics resembling a cylindrical bore at the bottom end.  It seems a minor issue, since you still have plenty of opportunity to develop your overtones with or without the weird result on low A.