Resolution:
To create a placeholder/binder on a task, use the API call
CIcTask_addBinder (or OLE Task Method AddBinder *Binder*),
as per InConcert Programmer's Reference Guide, page 25-5, or
InConcert Programmer's Quick Reference Guide for OLE Automation,
page 2-69.

To bind a document to a placeholder/binder, use the API call
CIcBinder_bindDocument (or set the OLE Binder Property Document),
as per InConcert Programmer's Reference Guide, page 4-3, or
InConcert Programmer's Quick Reference Guide for OLE Automation,
page 2-4.

Note you can also add binders to the Job.