Developers have already been using Composer's composer.json to enable composer to install their packages. This has worked relatively well. There is the problem of end users without shell access, but I hope that tools like NoConsoleComposer can address this.
The recent work on extension registration requires extensions to have another json file: extension.json. The problems this introduces have been discussed on wikitech-l.
Composer should be enhanced to understand the information that extension.json provides so that developers only have to maintain a single .json file.