I like the idea, but the implementation is a bit brittle. What about adding method hooks on ModelAdmin that would allow you to specify the options for a given ForeignKey field? These methods could get passed the request object, so you could do per-user options. Something like this:
# Model
class Book(models.Model):
title = models.CharField(maxlength=100)
author = models.ForeignKey(Author)
# Admin
class BookAdmin(ModelAdmin):
def dynamic_author_choices(self, request, book):
# Default implementation:
# return book.author_set.all()
return book.author_set.filter(use_in_admin=True)
