Lorna Mitchell has a quick post to her site today showing you how to use a GitHub branch as a Composer dependency when the need arrises for something other than master (or whatever branch is "stable" for the project).
My current project sees Celery (a python distributed task queue) added to my PHP application. There's a handy PHP interface to the RabbitMQ that Celery uses as a backend, which makes it easy for me to create jobs, called celery-php. This requires either the PECL AMQP extension< or alternatively it has experimental support for the PHP library for AMQP - I would normally prefer the PECL version but ran into version compatibility problems, missing manual pages, and decided that a pure PHP solution might be more portable and perhaps I would just add the experimental branch to my composer.json file for this project.
She includes an example of what the "composer.json" file would look like to pull this other branch. Two pieces of data have to be defined - the URL for the repository (to prevent Composer from trying to find it) and the branch name in the "require" section where the version would normally be.