I'm pretty sure I don't want to go down that road for an application that's meant to go into production (some day). It's possible to use Polymer elements in react. There's even a npm package[1] for it, but merely by looking at the activity of the package I'd already choose not to use this for a production application other than my own. Furthermore: I've tried Polymer and it offers its own data-binding implementation. I don't want to have multiple UI layers and two ways of data-binding. That's just asking for trouble.

[1] https://www.npmjs.com/package/react-polymer

Wouldn't passing things from react -> custom element be the same as to any other react component? Web components are like any other dom nodes - they should not conflict.