Hi!
Changing the messages in this way may be good from a performance perspective it really hurts the usability of our messages. Messages are localised at translatewiki.net and this is where we do require these messages. When messages are to have hardcoded strings as you propose you defeat the objectives of using a set of messages that are universally usable.
I'm suggesting doing that just on our wikis. Mediawiki users can have whatever expensive logic, I don't care that much :-)
In my opinion your proposal has really nasty side effects so my question is how you want to ensure that we are not working cross purposes.
Using {{ on common messages is no-go on major wikimedia wikis. Again, people can do whatever transformations they want at any level, except final mediawiki message logic that is on our site.
BR, Domas