I am a trained professional, don't try this at home!

models.py

class WeekDependentManager(models.Manager):
    def get_filters(self, prefix=None):
        if prefix is not None and prefix != '':
            prefix = '%s__' % prefix
        else:
            prefix = ''

        # All participating
        filters = {
            '%sweek__id' % prefix: settings.WEEK_ID,
            }

        # Only those extending
        extended_filters = {
            '%sweek__id' % prefix: settings.WEEK_ID,
            '%sextending' % prefix: True,
            '%sextending_end_date__gte' % prefix: datetime.date.today,
            }

        if settings.EXTENDED:
            return filters
        else:
            return extended_filters

    def this_week(self):
        # Don't change this!
        return self.get_query_set().filter(
            **self.get_filters(prefix='whatever')).distinct()

class RelatedModel(models.Model):
    objects = WeekDependentManager()

    whatever = models.ForeignKey('Whatever')
    ...


class WhateverManager(WeekDependentManager):
    def this_week(self):
        return self.get_query_set().filter(
            **self.get_filters(prefix=None)).distinct()

class Whatever(models.Model):
    objects = WhateverManager

    ...