No there is no workable Slackware version for PPC machines.
There is quite a few Linux distribution for PPC. Check out
http://www.penguinppc.org/ that is where most of the Linux distribution for PPC is "monitored".
If you are a Slackware user, I would recomend you
CRUX PPC; it has two version. One is just minimal installation, the other the called it evolution a fatter distribution.
If you don't mind trying out *BSD. I recomend NetBSD.