When hosting a web server, your app must listen on 0.0.0.0, adding that to your repl seems to make that work. I will make sure that is in our docs for web hosting.
Working Racket web app: https://replit.com/@ConnorBrewster/racket-server
Also played a bit with the Glitch import on an Elm import but found it broke npm in weird ways (something about a file being moved but not found?), but then the Github import of the same project wouldn't work either because of Node version incompatibilities that don't seem to be repl.it's fault exactly other than just "it runs node 12 and some stuff is broke on node 12".
Does look promising otherwise though, with the Nix support I can see soooo many things being possible that simply weren't on other similar options like this.