No, it can't be done in the OPF file. It's part of the EXTH header in the binary MOBI file itself. If you want to do it with calibre, conversion is the only option (EDIT: or I should say saving/sending as well). There's a Java metadata editor in the Kindle subforum that appears to be able to modify this without conversion, as well as the mobiperl tools, but I'm not exactly sure how much (if any) testing of KF8 files has been done.
Last edited by DiapDealer; 02-27-2012 at 12:09 PM.