1. Install/register WSA in your preferred location (I used MagiskOnWSALocal so it’s installer script takes care of that).
2. Make sure that WSA is actually turned off (e.g. WSA Settings → System → Turn off).
3. Move the userdata.vhdx & metadata.vhdx files from %LOCALAPPDATA%\Packages\MicrosoftCorporationII.WindowsSubsystemForAndroid_8wekyb3d8bbwe\LocalCache to somewhere else (if not present in the beginning then start WSA for the first time, then turn it off and check again).
4. Create symbolic links for them in their usual places. You can use Link Shell Extension to add context menu entries for this (Pick Link Source, then Drop Symbolic Link).