Create a hidden field in the admin site

How can I create a fully hidden field (input and label) in the admin site? I know about the exclude property, but it fully excludes the field from the template, while I need it in the web page, but hidden:

class OutForm(ModelForm):
    reply_to = forms.ModelChoiceField(queryset=InMessages.objects.all(), widget=forms.HiddenInput)

In the admin template I actually can hide a field, but not its label.