If you have gone down the route of creating your own custom image of Kali for the Pi you will need to enable SSH for yourself to make life a bit easier for experimenting with ideas.
Remove the SSH run levels first
update-rc.d -f ssh remove
Load the default SSH run levels
update-rc.d -f ssh defaults
Backup and move the default SSH keys and generate some fresh ones to prevent a MITM attack against your Pi.
Want a Message of the Day banner? Why the hell not! Install figlet.
apt-get install figlet
Usage is simply
figlet followed by words you wish to have modified
Just copy the text from the teminal and then open up /etc/motd and paste in your new message
Add in whatever you want here and save
SSH into your box to see your new message