After looking at the source, I've found that A-Frame uses
document.registerElement to register the
a-entity, etc, elements.
This confirms that we can use any DOM manipulation library like React, Mithril, CitoJS, AngularJS, Handlebars, Meteor Spacebars, etc, etc, etc. Those libraries will simply output to or modify the DOM, then the library that created the custom elements (f.e. A-Frame, or our own project!) can decide how those custom elements work.
This is genius, and will be the best way to integrate with... everything.
I'm surprised no one in the whole (what once was) Famous community (and now us) saw the opportunity to use
document.registerElement like this! GENIUS!