There is nothing illegal about riding there, unless used as a shortcut to undercut a rider...which in the case of Meares was not the case as she had made the move and then moved inside as Pendleton tried to crowd her out. If used to undercut another rider then this will result in disqualification.
However, during timed events there are sponges placed in the cote d'azure to ensure the minimum distance is covered.
I am not expert in the technical aspects of track cycling, althouhg in my experience stiffness is more important than weight in sprint events, although weight does obviously play a part as well -Meares probably finds the Rolls sadlle is comfortable.