Yeah you can totally do that. Mainly you'll need to create a second handler function and then use the website domain name to pick which handler to use for each request. Something like this, in your main namespace:

(def main-routes [["" {:middleware [mid/wrap-site-defaults]}
                   (keep :routes modules)]
                  ["" {:middleware [mid/wrap-api-defaults]}
                   (keep :api-routes modules)]])

(def alt-routes [["" {:middleware [mid/wrap-site-defaults]}
                  (keep :alt-routes modules)]])

(def main-handler (-> (biff/reitit-handler {:routes main-routes})
                   mid/wrap-base-defaults))

(def alt-handler (-> (biff/reitit-handler {:routes alt-routes})
                     mid/wrap-base-defaults))

(defn handler [{:keys [server-name] :as ctx}]
  (if (= server-name "")
    (alt-handler ctx)
    (main-handler ctx)))

Now all that being said, another good option for a static site is you can write out all the pages to html files on your local machine and then deploy them to e.g. https://www.netlify.com/ for free. That's what I do for http://biffweb.com and http://tfos.co. There is a decent amount of glue code required though, and since you're already familiar with biff development, going the netlify route would probably be more work with not much benefit. I do have another project aimed at static site development, but it's not documented yet and probably needs some more code work too before the first public release. https://github.com/borkdude/quickblog could be worth looking at though.

Wonderful! And Netlify is new to me. It's good to have options.

Finally got around to implementing this and it worked like a charm!