Kpdf is a pdfviewer and there is no need to pull in another pdf viewer like xpdf when one emerge a package that depend on "virtual/pdfviewer". I therefore propose to have the kpdf ebuild modified to include a line PROVIDE="virtual/pdfviewer"
Regarding KDE apps it should only be a matter how you've configured file associations. Regarding other apps however, all relevant apps would need to be patched to use e.g. a symlink to app X providing virtual V instead either app X, Y or Z hardcoded. Just inventing a virtual does not help anything. This is something you can ask for on the gentoo-dev mailing list, but I'm not so sure this is deemed important. So no - unless there's a Gentoo wide decision to go this way.
The "virtual/pdfviewer" is already in the portage. At least xpdf ebuild is providing it. Packages like lyx has a dependency on it.
Wasn't aware of it. Discussion is here: http://groups.google.de/group/linux.gentoo.dev/browse_thread/thread/894b903a4df38844/3c52933176a59ce1?lnk=st&q=questionable+usefulness+of+virtual%2Fpdfviewer%2Cpsviewer&rnum=1#3c52933176a59ce1