I see MythBox in my list of available addons for xbmc.
I know they have support for different pvr stuff, which I think is what you might be talking about. I'm not sure if the mythtv support conflicts with them or not, but here's a gentoo forum post about it. http://forums.gentoo.org/viewtopic-t-92 ... art-0.html
I also see unstable portage has an addon package available, but it looks to be very bleeding edge, so I can't say I recommend installing it, but if you wish to try, it's there... http://gpo.zugaina.org/media-plugins/xbmc-addon-xvdr