But can you have it in the Library module, so that when you have for example, four virtual copies of the same photo with different processing you can label each one with a short description?
Is Adobe's reason for not including these requests because the metadata would not conform to standards and thus would not be able to be displayed in other software?
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".
Edit:
http://tinyurl.com/IPTC-Standards
This is the latest IPTC Standard. It looks like Caption/Description is what you are looking for.