hi Simon,
to do a multilib setup, you have just to follow what's written here, upgrading glibc/gcc, installing the compat32-tools and the stuff in ./slackware64-compat32/
http://www.slackware.com/~alien/multilib/
after having done that, in the specific case of google-talkplugin, you don't need to follow what's written in the sections "Adding 32-bit programs", "Running 32-bit programs" and "Compiling 32-bit programs", but just use normally the
SlackBuild on SBo, that repackages the deb labelled "amd64" (don't get misleaded by that, it still needs multilib).