How did you disable telnet exactly - stop the service commenting out in /etc/inetd.conf ?

If it can run on the port then you could put a message in the login banner which appears
from /etc/security/login.cfg, which also then appears with other local logins (not CDE or SSH).
Alternatively you could edit the message catalogue for telnet, as you would have to with
FTP, to alter the login message specifically for telnet only.

If you've stopped telnet running altogether, then obviously you can't have a message
from it.