Just to make sure we're talking about the same fix, do you mean running the LAV installer first (edit: followed by selecting DirectShow in the options)? That worked for me with Proton 5.13-6.
Welp I got curious whether I'd missed a step so I gave it a once over, took me 7 hours to get it working. Turns out Proton does not like to run from an NTFS partition so I reinstalled steam on ext4. Now it runs but I couldn't get Direct Show to work, only the VLC video-path. I assume the LAV installer failed somehow. I also had to mount bind my external drive in my home folder otherwise it wouldn't see it. And I also had to reset whirligig's settings manually in the prefix folder (steamapps/compatdata/451650/pfx/drive_c/users/steamuser/Application Data/Whirligig/production) because it randomly stops accepting inputs.
Just writing it all out in case somehow comes across it.