Good question. In hexagonal.js approach client side is full app - so there's model part (use case is part of model), glue ~ controller and views are GUI and other adapters.
Thanks for letting me know. I'll not be rushing out to add hexagonal.js to my Rails app, but if one of my friends who likes js wants to try building their own full app I'll mention this as a possibility.
To be honest most of project I developed with hexagonal.js have Rails as a backend and it's really powerful with sprockets on board. So give it a try, maybe in toy project.