I don't understand. In Yosemite you don't need any UI Scripting. Have they removed this from El Capitan as well? What happened to:

   tell application "Messages"
      set iMessageService to (first service whose service type is iMessage)
      send "Hello World" to buddy "0123456789" of iMessageService
   end tell

No, that will no longer work in OS X 10.11 without disabling OS X's system integrity protection

Really? I just tried in the script editor and it works.

I also just tried it on 10.11.5 w/o SIP disabled and it works for me too.

Cool, didn't realize that, thanks for checking. However this method doesn't appear to work for group chats as far as I can tell, only one-on-one messages