Dynamically add Recipients to an Envelope at Send Time

I am implementing DocuSign with Salesforce for a customer.  We will use DocuSign on the Contract object in Salesforce.  I would like to be able to create custom object that would have a lookup to Contract so that I could dynamically add as many signers as I want to this child custom object.   I would add a Contract lookup onto this custom object.  I wanted to use the ‘Related Lists’ feature when I am on the ‘Add Recipient’ screen when setting up my template.   But it seems like that only supports a very limited number of related objects… 

Salesforce / DocuSign Apps launcher - I need to add a recipient from a related list of a Custom Object in Document Envelope template. | Community

Is there a best way to do this dynamically?  The client really does not know the maximum number of potential signers for a given Contract/Envelope.  I do not want to have to add 20-30 lookup fields to Contact on the Contract object.

Is there a best practice way to do this?  

I am very comfortable with Flows/Apex if a solution would involve using those within Salesforce.