Hi,
I have placed this in my LocalSettings.php:
$wgSiteNotice = "{{msg:A1}}";
In the wiki there is a [[Template:A1]] page which contains:
Go to "[http://externalsite/%7B%7BNAMESPACE%7D%7D:%7B%7BPAGENAMEE%7D%7D {{PAGENAME}}]".
The result of the above is that NAMESPACE gets replaced by Special and PAGENAMEE gets replaced by Error, just like PAGENAME, regardless of the page I am looking in my wiki. So the result is that I always get Special:Error. If I remove {{NAMESPACE}} I just get Error.
I use mediawiki1.4beta4, unmodified.