Is it possible to mimic WLM's "local" messages that appear in chat window, like ones for undelivered messages or file transfer stuff?

No, this can't be done with a script.

Okay, is there a way then to add a message to Plus! logs without actually having it in conversation? I mean, of course it is possible as these are just external files; but I don't know how exactly MP!L uses them.

Not really.... Plus! logs aren't always updated after each and every text you send or recieve, so you can't simply add something to the existing text files directly.

As for doing something like the 'local' error messages in conversations. It could be possible with pure scripting but it would require soft patching and poking in the memory of Messenger. Not something easy todo (if possible at all...but I think it is), and no garantuee that Plus! will propely reconize it and save it in the logs.

EDIT: Update: Pathou has said he will think of something to add in regards to this. So it might be possible in future Messenger Plus! versions.

I see. I was trying to find a better way to show/save results of "secret" ("local") dice rolls in a script [better than a toast and no save at all], ideally integrating them into chat timeline. Event log can possibly do the thing, but it's not connected to chat. Another possible solution is to keep a separate log by the script, but it's too wasteful.
Thanks for your opinions/answers.

With msg+ you can't do for now, but it is possible,  but I dunno if it can be done with the script...ther are some other extensions for msg+ wich can copy error messeges, action messege etc...