To use Composer with free plugins on wordpress.org (where MB Rest API & MB Relationships are hosted), please use https://wpackagist.org/. It allows you to bundle any plugins for Composer.
Updated: I’ve just written a guide on doing that. See it here:
It has a video tutorial in case you prefer watching!
@Dave: Thanks for your suggestion. That’s what we talked about on Facebook :). I really appreciate that. We talked about making a customizer, where users can select what they want and it will generate a package for that. Yes, I guess it’s possible and it’s on my plan. What you say about the AIO is true and let me try if I can bundle free plugins inside that.