title: Dynamically change field of Django admin inline form
body: <p>I want to dynamically change a CharField to a ChoiceField, because I want the value to be selected from a list of possibilities that is determined by the current request.</p>
  
  <p>In a ModelAdmin I can do that with get_form(), and just say:</p>
  
  <pre><code>form.base_fields[field_name] = forms.ChoiceField(...)
  </code></pre>
  
  <p>but how can I do the same for an InlineModelAdmin (TabularInline)?
  I stepped a bit through get_formset() and get_fieldsets() but can't find the right spot for hooking in.</p>