title: Limiting Admin Choices Using limit_choices_to
body: <p>I would like to limit the choices for a ForeignKey in the admin UI using limit_choices_to; however, I would like to achieve this without changing the model, since the model is brought in from a library, that I don't have control over. What is the way of dynamically achieving this? Or could I use a field on the admin model to be able to achieve this?</p>
  
  <p>Thanks,
  --Eytan</p>