It's not clear how you're referencing the custom object as you can't send email directly to the custom object.
You should be able to use the FindObject function from Interact or Custom Actions and then you are are not restricted to ID,
or, if you set your Custom Object's ExternalID appropriately with a Workflow Rule/Custom Action, you can use the GetObjectByExternalID function which was designed for this kind of scenario. GetObjectByExternalID is available for use in Workflow Rules when FindObject is not available.