You are right, and I sit corrected. Though I am sure I read that somewhere when I got my piggyback FAA licence.
FAR 61.75 (c) states
(c) Aircraft ratings issued. Aircraft ratings listed on a person's foreign pilot license, in addition to any issued after testing under the provisions of this part, may be placed on that person's U.S. pilot certificate.
(my italics)
which does indeed indicate that you can have an IR added to a piggyback license.
Tim