Custom filterspecs for auth.user

All the techniques I've seen for adding custom filterspecs to the Django admin involve editing models.py (eg link text)

I'm customizing auth.user by registering my own Mode开发者_开发知识库lAdmin and would like to add some custom filterspecs. I could fake filterspecs in the template but I'd rather use custom filterspecs. Is there any way I can monkey-patch auth.user.models or achieve this some other way?