Earlier: Is there a way to change the current, contemporaneous default for ALL subsequent visitors, including registered and unregistered visitors?
Response: ... we have manually set user preferences for both new and existing users using a SQL REPLACE() statement combined with LocalSettings defaults. This in effect brings every user to the same settings... use an UPDATE... REPLACE SQL query to bring all existing users to the same settings.
Peter Blaise responds: I'm always happy to learn more about the inner workings of the support programs for MediaWiki software. What would such a query look like, how about an example? How about putting it somewhere at http://www.MediaWiki.org/ and linking to it? Thanks!