It might be an issue with relative vs full paths, but to be honest I am not sure whether arbitrary loading from any folder is allowed. Although you are loading from UserDir() value, so it should work.
Take a look for example at SpeedTree importer, they are loading textures and seem to have some path conversions: Engine/Plugins/Editor/SpeedTreeImporter/Source/SpeedTreeImporter/Private/SpeedTreeImportFactory.cpp