You can use your own subclass ModelFormfor the administrator by adding an attribute help_textfor the corresponding field when you initialize it and configure it accordingly.
class YourModelForm(forms.ModelForm):
def __init__(self, *args, **kwargs):
super(YourModelForm, self).__init__(*args, **kwargs)
self.fields['field_in_question'].help_text = generate_warning()
class YourModelAdmin(admin.ModelAdmin):
form = forms.YourModelForm
class Media:
css = {"all": ("admin_warning.css", )}
js = ("admin_warning.js", )