WS and CRMOD object associations

I am looking for advice about how the community is handeling reacting to events when a user associates two CRM object together.

The specific objects that I am having trouble with are Custom Object 2 and contact. (But I think this applies to most of the objects in CRMOD)

When my user's associates a contact with the Custom Object 2 I cannot figure out what I can trigger off of to react to this outside of the CRM application.

I don't seem to have access to the association modified timestamp like i do in the account/contact/role relationship.

I cannot create an Integration Service event since its not the parent object.