The maps cannot overlap. You could mount filer:/path/to/special somewhere else an make a symlink from /data/special to it. That symlink will be visible to all clients however.
A cleaner though more involved approach would be to use an executable map. The map script would be something like:
if [ "$KEY" = "/data/special" ]; then
echo " filer:/path/to/special"
ypcat -k my_nis_map | egrep "^$KEY\s+" | sed "s/^$KEY//"
(Warning: Script typed off the top of my head. NIS map musn't use \ for line continuation. I do not have access to NIS. May contain peanuts.)
Edit: http://phaq.phunsites.net/2008/01/24/an-autofs-executable-map-to-automount-device-nodes/ gives an example of an executable map.