You certainly can't have any mention of gifts printed on your invites, although most people aren't bothered by a little slip of paper naming the stores of registry. (parentingsquad.com)
Have them write their responses on a small slip of paper; use that paper to give a quiz grade. (educationworld.com)
However, etiquette dictates that, at the very least, registry information should be on a separate slip of paper, not on the invitation itself. (pregnancy-info.net)