Just as an update to my Upgrading KDE 4.2.4 to KDE 3.5.10 in Slackware 13 post, Patrick Volkerding has apparently released KDE 3.5.10 for Slackware 13.0 (link here).

This is probably a better way to go than my procedure, which was more of a proof of concept than an actual workable solution, though I have received some comments that indicate that the KDE 3.5.10 packages from Slackware 12.2 were usable in Slackware 13.0.

This quote from the README is actually one of the things causing me to feel validated in seeking another distro going forward though:

rest here