One thing that concerns me about the proposed composer setup, that I haven't mentioned yet, is the nesting of the project hierarchy, with mediawiki/core/vendor inside mediawiki/core.
You know that we have mediawiki/extensions instead of mediawiki/core/extensions -- that makes it easy to check out all Gerrit repos in the natural directory hierarchy and to still have a clean $IP/extensions which you can use for installer testing or whatever.
I wonder if a similar solution is possible with vendor -- could we have mediawiki/vendor instead of mediawiki/core/vendor? Then it would be possible to play around with dropping things into $IP/vendor while still having the "vendor" git repo checked out in a sensible location, available for editing.
-- Tim Starling