Whirlybird: spot on... I vote for your suggestion re- lane frequencies!
WWW: as per slim_slag, what you fear did occur in LA areas (OK, San Diego perhaps...) and this led to introduction of the 30NM mode C veils surrounding Class B over there
AFAIK.
Capt. Manuvar: there are already insufficient codes so no scope
AFAIK to use last two digits as you suggest. Also it would be very prone to human error given that we are already capable of forgetting to put the thing on or in mode C at all
!
Andy