generate Table of Contents from placed InDesign document (data merge)

I'm designing a small catalogue.

I'm just getting to grips with data merge, and I've designed a document which pulls the images and data from a spreadsheet into a placeholder document to make a whole load of "data boxes".

I then place the pages from this MERGED document into a new indesign document which has the background and footers for my catalogue (around 8 boxes on a page)

I want to use data inside of the placed document to generate my ToC so I have an index page. But I don't think inDesign can "see" into the placed document.

Any amazing ideas? I'm sure there are better ways to do this.

I'm using InDesign CC

Ruth