I agree in principle that it would be nice to have Discourse packaged with NixOS.
Gitlab is quite similar in that it’s a Rails-based project that also needs a bunch of dependencies like Nginx, Postgresql and Redis to run properly. Gitlab is distributed in nixpkgs and has been quite maintainer-heavy. Every ruby or gitlab release tends to break things as they make too many assumptions about the system and it’s packaged as one big tangle of files.
Until they see the light the more practical approach would be to run the Docker container which is well tested by their QA. Maybe it’s possible to run that image in the nixos-containers? Both services could probably still benefit from the declarative approach for the configuration files and the container setup.