I have been contemplating posting a welcome message to new users, in the comments of their question or answer. Ideally it would stay there for a few days, a week at most, then be deleted. Currently we don't have enough traffic for this to be a significant body of work. I previously mentioned something similar and did not get any volunteers so I plan on managing it myself.
A comment can be 600 characters, so that the longest it could be. I think the audience would be any new user with a first post and less then 100 rep (excludes people with other SE accounts and experience).
If you have suggestion for a new user welcome message, please post it as an answer. It should include links to anything you think is important for new users to read.