We can edit message wall greetings...
First copy this link and change the part with my user to urs
Then move page
Again replace my username with urs
Then change type of page and turn it into a template.
Now you can edit and when you are done change it back to a message wall greeting!
I hope that helps!
(for me it wont let me edit message wall greetings from using the first link, the editor crashes, not sure why. If it works for you, well that's great!)
DON'T CREATE MESSAGE WALLS. PLEASE, that's wrong-