Even so, for the message to be most effective it should be in my opinion in real-time, upon registration, and not a deferred PM. In addition, the potential PM sending account, being non-official, could be considered spam technically, so the more official the solution the better really.
Edit:
<…> I think you overlooked the fact that said account can have 30 recipients per PM, making it possible to reach 36000 users per day. <…>
No, I wasn’t aware until now. Still making it a trigger or alike sort of feature thar sends a PM and/or displays a pop-up seems rather simple. The issue I figure is not implementing the message, but rather more deciding the content of the message.
I agree, but until something is implemented by the Admins anything is better than nothing our message may not be perfect but could still be effective. The "unnofficial" was in line with the labeling of the rules we currently have (Unofficial list of (official) Bitcointalk.org rules, guidelines, FAQ) and for complete clarity on that this is a community message - which might be a good intro that some systems within the forum are community driven as opposed to moderated. I don't agree it could be labelled as spam as long as the content adds value.
With most things if it wasn't to hard for someone to implement, go for it. If theymos doesn't like it he will speak to it, or ask the individual to stop. There also would be nothing preventing theymos from implementing his polished message if/when it's complete. Then there would no longer be a need to continue this interim measure.
Of course to complicate matters a notch, ideally it should be available in the language of the user, either first hand (i.e. when subscribing - select language for welcome message) or accessible later on (which loses efficiency). Default English is perhaps good enough, although many non-native would not grasp the depth of the content, but rather more a watered down version of it's content.
For local board languages it may be worth just posting links at the top of the message in the languages we currently have local boards for. As time goes on I'm sure someone would translate the message from each board and it could be linked to that post.