Capybara test reverse proxied domains

I'm currently working on a rails application which has an interesting setup.

We have two applications running on heroku. One is a basic app which works as a reverse proxy, it has multiple domains attached to it and it reverse proxies to our main application which is a rails application ( this application does a look up based on the host then for the routing ).

The reason for this setup is to allow custom domains pointing to "shop pages" on the main application, these shop pages then point back to the original domain for a checkout.

Maybe an example will better explain it, 2 domains: seller.com platform.com

platform.com represents the app host for the main application, seller.com is a domain routed through the reverse proxy. On seller.com you choose your products and then ultimately you check out on platform.com

We're struggling to find a way to test this end to end flow in our main application using capybara given the multiple domains, is there any way to handle this by spoofing the reverse proxy or just using multiple hosts?