Attaching row-level signals in django?

Question

I have two models, a definition:

class Indicator(TimestampedModel):
    some_definition_data = models.IntegerField()

and an instance:

class IndicatorInstance(TimestampedModel):
    indicator = models.ForeignKey(Indicator)
    some_instance_data = models.TextField()

I want to attach event code (signals) to Indicator definitions. I suppose a signal attached to Indicator with a bunch of if statements checking the Indicator type could suffice; however, I was hoping for something much better. Can anyone offer any advice?

Thanks, Pete