I have this same need. I think that this import/export custom ribbon functionality is only truly portable if it uses relative paths. I'm sure if you are just grouping common command buttons on a ribbon tab, this works great. But for script tools, it means we can't share them. I would love to be able to point some buttons to script tools within the project toolbox and create a ribbon for accessing them.
Another awesome thing would be if you could select custom ribbons to include when sharing a project template.