I looked and the key provides seems to be system-backgrounds-gnome. That is
provided only by laughlin-backgrounds-gnome and is required by
gnome-desktop and gnome-desktop3.
I didn't check for file dependencies, so I am not sure if XFCE pulls this
in via a file dependency or not.
But it does look like we need a package change to deal with this.
One option would be to have laughlin-backgrounds-single provide
system-backgrounds-gnome instead of laughlin-backgrounds-gnome and then
specifically include laughlin-backgrounds-gnome in the ks files for
the spins that want the full list of backgrounds.
I'd be willing to adjust the spin kickstart files, but I am not a proven
packager so I can't touch that laughlin-backgrounds packages. We'll also
need to round up some proven testers as these are critical path packages.
I have about 2.5 hours now I can be free to do stuff, otherwise it will
be late tonight (in about 14 hours) or tomorrow that I can work on this.
Another option would be to revert the update. That doesn't seem ideal,
but might be less work.