I wish I had seen this message sooner, sorry!
A good trick I've been using involves merging without the common artwork in place, then putting it onto the merged document's master page.
1. Save a copy of your full layout as, say, "Art-master.indd"
2. Using another copy of the file (as a precaution), perhaps named "Merge-master.indd", delete all objects that are not part of the data merge. e.g. your postcard background image, any other static items placed as part of the layout.
3. Perform the data merge on this pared-down document. It should run faster, too, than with the other objects in place.
4. In your merged document, switch to the master page and, from your Art-master document, copy then paste-in-place those static objects. The art should now appear on all merged pages in the document, and linked once!