populating Multigroup fields from header node id

I'm building a View which is a summary of several nodes of a particular content type.
There's some header info, and then a Multigroup with a couple of fields for each item in the (selected) list. One of the header fields identifies which nodes are being listed.
I want to populate the list from the header entry. Right now, I'm looking at Validation to do it -- I can do the lookups and get the data I need from the node id with no trouble. But the /node/add/foo-summary page gives two sets of blanks with an "Add more Values" button below them.
How do I (mimic the action of the button) add as many more sets of fields as I need?

Or, is there a better approach entirely?