Anyone know  if it's  possible  to  stop  movement  of  an  object  in  the  Z  plane  of  a  3D  model  when  using  snaps  to  line  objects  up ?

There is the OSNAPZ variable ("ignore Z") in higher AutoCAD versions. See:
http://www.cadforum.cz/cadforum_en/qaID.asp?tip=4286

Cheers, just what I wanted