I am by no means a composer expert. In fact it was something that Ale at Briteweb introduced me to a little over 9 months ago. So whilst the method that I’m describing below most definitely works – and is stable and solid – it is by no means the only, or indeed possibly best way to accomplish this task.
Composer packages, by default, are placed into their own directory, within the ‘install path’ that you specify. If you use the wpackagist repository then you have access to the wordpress-muplugin type. Unfortunately, WordPress Must Use (MU) plugins – by default – must be in the root of the mu-plugins directory (which by default is mu-plugins in your wp-content directory, but can be defined using WPMU_PLUGIN_DIR and WPMU_PLUGIN_URL ). This means that, without any modification, any composer package that is installed into the mu-plugins directory simply won’t work. Bummer.