Can you not just send the users request to two different URL, based on the decision if it needs auth or not? That would not be authelias task but your internal url routers. Authelia would handle the authentication on the path that requires auth and your proxy would directly serve the “open” content. The logic would have to take place in your app.
myapp.org/ checks what it’s going to be
myapp.org/secret/ serves content you need to be authenticated for
myapp.ort/open/ serves content that doesn’t need auth.
I think that authelia is not really made for this scenario, so if you want to use authelia, I think you would have to loop back to before authelia and redirect to the different (auth / non-auth) urls.
I am pretty sure though you are not the first person to have this requirement so probably there is a better solution, but I would have no idea what to search for.
Generally speaking most proxies like haproxy or nginx are scriptable (HAproxy via Lua for example) so maybe that’s something you could let ok into. This article sounds s bit like your scenario:
https://www.egnyte.com/blog/post/dynamic-backends-in-haproxy-with-lua