RetroPie doesn't have a Doom Legacy package - it's probably something extra included. I don't find any information about the RGB-Pi OS being based on RetroPie (though the documentation has some info that include RetroPie specific paths), nor a source repository for the image - maybe ask the image author about the issue ?

@Solid-One That works! The Port itself doesn't seem to work very well yet, and I only have Doom Shareware as an option despite having all the WADs in place in the Doom directory. But I haven't looked at this properly yet, and the change to the location within the Script Module worked a treat. Thank you!