XBMC depends only on unrar, but there is also unrar-gpl which provides the same functionality. From the mplayer ebuild: rar? ( || ( app-arch/unrar-gpl app-arch/unrar ) ) XBMC ebuild only checks for unrar, resulting in blocking errors if you have unrar-gpl installed. I don't know if this qualifies for a virtual, but since they basically provide the same functionality, I would suspect so.
http://sources.gentoo.org/media-tv/xbmc/xbmc-9.04.ebuild?r1=1.1&r2=1.2 http://sources.gentoo.org/media-tv/xbmc/xbmc-9999.ebuild?r1=1.24&r2=1.25