I am looking for a way to install WPMU and a premium ACF plugin with composer, but have not found any functional way of installing them. The best alternativ I found is to upload them to a private bitbucket and then install them with Satis. If someone could point me in to a practical example and/or a better alternative then Satis.
Composer inherently prefers packages that support it explicitly. It is flexible enough to still work in other cases, but it gets less pleasant.
The options used in practice typically are either including premium plugins into version control or re–hosting them yourselves.
After yet another discussion on topic I have also created Release Belt project for this. It aims to easily serve ZIP archives as Composer repository. It isn't much used/tested so far though.