Everything so far is working perfectly by first upgrading GSB to 2.28 with a local mirror and upgradepkg and then using slackpkg to upgrade to 13.1 after blacklisting the GSB replacement packages.
The only hang up that I have found was that after the upgrade to current/13.1 I could no longer change any of the backgrounds or themes in gnome until I created a symlink to a library that got upgraded:
dtest@dtest:~$ ls -alt /usr/lib/libltdl.so.3
lrwxrwxrwx 1 root root 19 2010-05-07 21:27 /usr/lib/libltdl.so.3 -> /usr/lib/libltdl.so*
Since that I have yet to find anything that doesnt work.