By putting smitty mksysb as your default shell in passwd, by pressing F9 you are escaping to your default shell, whenever you press Enter you will be redirected back into the smitty mksysb menu. If pressing any other key, (such as control characters Ctrl+Z), you will be disconnected. Smitty mksysb cannot interpret these commands.
call: As far as creating a user to execute just mksysb you have 2 examples here. I can't think of another way, although if you do not want the user to be able to break out of executing their .profile, you could read in the users id from the login (in /etc/profile $LOGNAME) and execute the same scripts if that user logs in.
trap "" 1 2 3
readonly LOGNAME