> Can you please elaborate on "This is also not guaranteed to work or will still work in future versions of MediaWiki", in what scenarios it is not guaranteed to work?.
The problem is, that the LocalSettings.php file is evaluated while MediaWiki is setup, so even if the User object is already available and fully working in the current version of MediaWiki, no one can guarantee, that it will work in a future release of MediaWiki, because, maybe, the initialization process changed or something else. That's why I wouldn't suggest to rely on this.
Btw.: From what you write, I would think it will be better to have different wikis for each of your brand. I'm not sure, how you want to substitute variables in MediaWiki, and even if it's possible I'm pretty sure, it will not work as you expect because of the internal caching of parsed wikitext, which is integrated in MediaWiki for performance reasons. I also would say: Different site logo + different content = different site, so you should use one MediaWiki installation for each wiki.