Got this running, though there were a couple odd issues along the way. Noting it here for anyone else:
- I made the mistake of running the game's exe in Windows and then transferring that whole folder to my Deck. I believe the game does a compilation process when you launch it on Windows, so it'll clear out files from the content folders, meaning it won't be able to find those files when you run it from Proton and it'll throw up a missing files error on launch. Use a fresh version of the extracted folder
- For whatever reason, the game launched with the sound muted for me. If this happens, just right-click the application in the taskbar and adjust the audio
Thanks for the instructions, by the way.