Answering my own question: I did some playing - turns out slackware uses a bsd style rc.M script normally but also has an rc.sysvinit script for sysV compatability. I didn't want to get into playing with the rc.M file (looks like just cutting and pasting the proper start/stop sections from the postgresql.txt file into the appropriate place in the rc.M file) so I read through the rc.sysvinit file to figure out where to put the K20 and S20 symlinks to /etc/rc.d/init.d/postgresql - first you have to create the /etc/rc.d/init.d directory, then copy postgresql into it. Next, create the symlinks as described in the Suse section of the documentation (ln -s /etc/rc.d/init.d/postgresql K20postgresql, then S20postgresql). Next, create rc2.d, rc3.d, rc4.d, rc5.d directories in /etc/rc.d. Finally, copy the K20 and S20postgresql symlinks into each of those new directories. et voila - it worked. Probably not the most elegant way for Slackware but until I learn a bit more...
