I spent my weekend on this and I've got it roughly working, but I haven't tested it very much because I never actually use the Home-Cooked Levels menu myself, so I'm not the best person to make sure everything is working properly. The only thing changed here compared to regular 5.12 is the Home-Cooked Levels menu, this should be fully compatible with all other uses.

One unfortunate detail is that I had to disable saving for levels loaded from subfolders, because there's no way to include that information in a save file.