Well, to list the available Linux partitions to serve as targets the 'setup' script runs the 'probe' script:
if probe -l 2> /dev/null | egrep 'Linux$' 1> /dev/null 2> /dev/null ; then
probe -l 2> /dev/null | egrep 'Linux$' | sort 1> $TMP/SeTplist 2> /dev/null
dialog --title "NO LINUX PARTITIONS DETECTED" \
--msgbox "There don't seem to be any partitions on this machine of type \
Linux. You'll need to make at least one of these to install Linux. \
To do this, you'll need to leave 'setup', and make the partitions using \
'cfdisk' or 'fdisk'. For more information, read the 'setup' help \
file from the next menu." 10 60
The 'probe' script is "a wrapper for using fdisk to gather drive info for the Slackware setup scripts."
So I suppose that to do what you want you would need to modify the installer itself. Good luck.
But maybe I am wrong and somebody will propose you an easier way ?
I append /sbin/probe found in the installer for Slackware-almost14 though that be probably useless.
PS maybe it's possible to set up an initrd including the whole tree of a complete Slackware installation, then cp it in a directory. But that would need a lot of RAM and it's probably simpler to use rsync to mirror an existing Slackware installation into your target directory then. But what about /dev in that case?