Hi,
The point is, then it won't get automatically updates from translatewiki.
Enwiki will be really really sad by not have pagetitle constantly translated by translatewiki :) On the other hand, one could support branching of messages at translatewiki - isn't that what we do with mediawiki already? :)
I don't support having a different syntax for mediawiki messages, but {{SITENAME}} can get susbstituted with a simple preg_replace (a str_replace if we know there won't be tags).
Can we avoid obfuscating our message system even more?
Domas