Common Data Service Connector is not leveraging CDS capabilities for alternate keys. Apart from GUID, Get Record action should accept alternate keys as well. It's a very convenient and clean way of pulling a record from CDS.
Why do we always find these things out the hard way!
It's really messed my plans up.
Great idea we need more votes!
In the meantime I think I'll build an intermediate entity which I can refer to and retrieve the contact ID