This is how I've gotten around the user name length limitation. I have a longer field in the subclass, and implement a backend to check that. But because it's a subclass, foreign-key relations still work.
For example, my typical use case: unique email address for logins, no usernames:
I suppose you could subclass User with a longer username, and sync the username with the email address using a signal, for example.
Still, it's a chunk of workaround I end up doing one way or another with each new project. Hopefully soon (based on discussions on django-developers group) we'll see some kind of pluggable User model.