Extending South Introspection in Django

I have a custom app which I wanted to start using South with. It uses a FK for associating a blog entry from another app (proprietary, not written by me). Problem is when I try to run the initial schemamigration I get an error that some of the blog entry app's fields cannot be frozen. The fields that can't be frozen are fields that uses custom fields extended off of the core fields.

./manage.py schemamigration free --initial

I read South's docs on extending introspection but I don't know where/how to define the introspection rules.

Thanks in advance for any suggestions.

Have you tried the South Field Triple approach instead?