I have continued to work on this and have added a new setting for Kodi's listed aspect ratio.
My issue with this setting, and the reason that I didn't go this route originally, is that I don't know if it is correct. Unless you use TMM as @hslansky suggested or have your videos cropped and recompressed like @Epitaph's list, they will all be 1.78 : 1 as that's what a full frame blu-ray with black bars is.
I have made the assumption that if you use nfo files, Kodi will have already scraped the data from them and we can just use that. So there is now a sub-setting for this in PreShow to select how you want it handles.
I'm not sure if that's the right wording, but Kodi has a character limit on the settings, and to explain it properly requires scrolling.
And then I've modified the IMDB selection option to give the user some more control. Originally I tried to make it so that it would pick the most likely option for movies that list multiple options. That worked for the majority of cases, but not all. There are some that aren't labelled correctly, and then there's always the multi-aspect movies. Now you'll get a pop-up so that you can select which one you want.