We don’t need a special term for interoperable.

A software system can be fully interoperable without any part of it (other than the interface) being open. In my definition, an Open Shell software solution is one in which a company makes it clear that they're focusing on building and selling (or making money from hosting/supporting) only the "Core" functionality, while exposing all of the higher level functionality as open source plugins and fostering a community to make it as easy as possible for others to improve these and maintain them over time. And preferably, the company would also pledge never to offer its own closed source functionality outside its definition of the core.