Thanks for the reply, Alberto.
Yes, to my surprise, Messages is compiling and executing its own AppleScripts.
My user was actually trying to implement a screen sharing session using the auto accept script, but as I discovered after a bit of digging, that was only ever available in iChat:
This handler didn’t get included in Messages, so problem ‘resolved’ in the sense that, it can’t be done.
Sorry, I don’t have any insight on the problem you mention.