models.py

class Channel(MPTTModel):
    # ...
    tags = models.ManyToManyField('tags.Tag', blank=True, through='RankedChannelTag', related_name='ranked_tags')

# ...

class RankedChannelTag(models.Model):
    channel = models.ForeignKey(Channel, on_delete=models.CASCADE)
    tag = models.ForeignKey('tags.Tag', on_delete=models.CASCADE)
    rank = models.SmallIntegerField(default=0)
    # ...

serializers.py

class RankedTagSerializer(serializers.ModelSerializer):
    rank = serializers.IntegerField(required=False)
    tag = serializers.CharField(source='tag.name')

    class Meta:
        model = RankedChannelTag
        fields = ('id', 'channel', 'tag', 'rank')
        read_only_fields = ('id',)

class ChannelSerializer(serializers.ModelSerializer):
    tags = RankedTagSerializer(source="rankedchanneltag_set", many=True, read_only=True)
    # ...

# Serializing a channel throws: 
# AttributeError: 'ManyRelatedManager' object has no attribute 'pk'

# Serializing a RankedTag works just fine.