Editing registry keys in a Proton prefix?

I’m trying to get the new Halo 3: Project Reclaimer mod up and running on my PC, which already has Halo: MCC installed and working properly, and I came across this guide and script to get it running on an Ubuntu machine. Part of the setup involves adding some registry keys to a Proton prefix, but the script provided in the guide obviously doesn’t work on a non-FHS distro like NixOS: export WINEPREFIX="$HOME/.steam/steam/steamapps/compatdata/976730/pfx" WINE="$HOME/.steam/steam/steamapps/commo...