I've lost my link to the IPTC webpage, but I think there already is a "Comments" field. At least in Vista, the Lightroom "Caption" field maps to both the "Comments" and "Subject" fields in image properties. To make matters worse, "Caption" shows up in the "Description" field in Pro Photo Tools. The only one I have seen that is consistent from application to application is "Title".
This is the latest IPTC Standard. It looks like Caption/Description is what you are looking for.