I agree with @apertureless. Designers should also have basic understanding of how React components work. This will speed things up and help developers and designers collaborate in a better way. The following workflow may work, but I would love to see what others recommend :
render() function that returns a dummy markup) and pass those to designers.render() and do whatever they want. If designers write HTML separately and developers write corresponding React components, it may introduce additional complexity while introducing changes. Doing everything in components is the right way, imo.