I've had reasonably good success by lifting updated packages from Slackware-oriented Gnome installations. For example, to get gtk+ 2.8.x, I downloaded and installed gtk+2-2.8.6-i486-1frg.tgz from the Freerock Gnome current fileset at http://mirror.datapipe.net/norlug/fr...ome/platform/.
I notice that there are also cairo and other such things available.
I usually scan the package filelist from the official Slackware package that I already have installed, in case there's something there that I wouldn't want to tamper with by updating.
I'd also make sure you have the official Slackware packages on hand in case you need to revert, but so far, I haven't caused any major damage using this approach. I'm sure you could overdo it, however, and hose something critical.
But, hey, don't you like the adrenaline rush that comes from having to try to restore a crippled Linux installation (if you're in a production or server environment, you could probably do without, I'm sure)?