Creasy,
The ISAGO standards require an accredited ground handler to have an Emergency Response Plan however doesn't provide a lot in the guidance material as to what it should comprise. What it does allow however is recognition of the airport or operator ERP and for the ground handler to point to that.
One aspect of this question is does it relate to an emergency within the ground handler itself, or the ground handler assisting in the response to an operator/airport incident.
Ground handlers I have seen don't do so well at the former, and their strength at the latter depends on the level of training and exercise training from the airport/operator.