def mark_payment_as_sent_action():
    ''' Admin action to mark payment as sent '''

    def mark_payment_as_sent(modeladmin, request, queryset):

        # #####################################################################
        # This is what I currently do
        payments = queryset.filter(status=models.Payment.S_PENDING)

        # This is what I want to do
        payments = queryset.pending()
        # #####################################################################

        # Do stuff with filtered payments

        return HttpResponseRedirect("...")

    mark_payment_as_sent.short_description = "Mark as sent"
    return mark_payment_as_sent

class PaymentQuerySet(models.query.QuerySet):

    def pending(self):
        return self.filter(status=self.model.S_PENDING)

class PaymentManager(models.Manager):
    use_for_related_fields = True

    def get_query_set(self):
        return PaymentQuerySet(self.model)

    def pending(self, *args, **kwargs):
        return self.get_query_set().pending(*args, **kwargs)

class Payment(models.Model):

    status = models.CharField(
        max_length=25,
        choices=((S_PENDING, 'Pending'), ...)
    )

    objects = managers.PaymentManager()

@admin.register(models.Payment)
class PaymentsAdmin(admin.ModelAdmin):

    actions = (
        admin_actions.mark_payment_as_sent_action(),
    )