As far as I know, using tag files is the closest you'll get to 'kickstart'. I'm unaware of any functionality that lets you predefine drive slice setups. I'm also unaware of something that allows you to define scripts to be executed, post installation.
Considering slack does not use a partitioning program like diskdruid, which may be designed to implement predefined slice setups, I could see how this could be harder to accomplish via fdisk, if even possible at all.
One of the other slack users may know of something I don't mind you.
Good choice on going with Slackware btw!