I think you are going to need some customizations. At minimum, I think you need to subclass ClusterTaggableManager so you can pass an ordering through to Taggit. Personally I would start there and test it passing whatever you need to change the ordering to alphabetical by tag name. I know that isn't what you want, but it gives you a test you can do without needing any changes to the UI. Once you have that working, then I would figure out how to tell the manager what order you want.

Does the many to many table even have a sort column? (I would look directly in the database to verify this)