¿Cómo puedo configurar en ModelChoiceFieldgrupo opcional ¿etiqueta?

Este es un ejemplo:

modelos.py

class Link(models.Model):
    config = models.ForeignKey(Config)
    name = models.URLField(u'Name', null=True, max_length=50)
    gateway = models.IPAddressField(u'Gateway', null=True)
    weight = models.IntegerField(u'Weight', null=True)
    description = models.TextField(u'Description', blank=True)

def __unicode__(self):
    return self.name

forms.py

class LinkForm(ModelForm):
    config = ModelChoiceField(queryset=Config.objects.all(), empty_label="Choose a link",widget=GroupedSelect())

class Meta:
    model = Link

Me gustaría hacer mi ChoiceField así:

example.html

<select id="id_config" name="config">
    <option selected="selected" value="">Choose a link</option>
    <optgroup label="Configuration" >
        <option value="8">Address: 192.168.1.202/255.255.255.0 </option>
        <option value="9">Address: 192.168.1.240/255.255.255.0 </option>
        <option value="10">Address: 192.168.3.1/255.255.255.0 </option>
    </optgroup>
</select>

**ACTUALIZAR**

Resolví mi problema así:

class GroupedSelect(Select):
    def render(self, name, value, attrs=None, choices=()):
        if value is None: value = ''
        final_attrs = self.build_attrs(attrs, name=name)
        output = [format_html('<select{0}>', flatatt(final_attrs))]
        for index, option_gp in enumerate(self.choices):
            if index == 0:
                option_value = smart_unicode(option_gp[0])
                option_label = smart_unicode(option_gp[1])
                output.append(u'<option value="%s">%s</option>' %  (escape(option_value), escape(option_label)))
                output.append('<optgroup label = "Configuration">')
            elif index!=0 and index <= len(self.choices):
                option_value = smart_unicode(option_gp[0])
                option_label = smart_unicode(option_gp[1])
                output.append(u'<option value="%s">%s</option>' % (escape(option_value), escape(option_label)))          
        output.append(u'</optgroup>')
        output.append(u'</select>')
        return mark_safe('\n'.join(output))