Yes I know this post is 4 months old but figured that someone would ask the question again...
Okay, I've been using slackware for quite some time, there are a few methods of madness to coax it into installing off of strange media, or lack ther of.
Firstly, a great tool to help "bootstrap" this stuff is Knoppix. Knoppix might detect and understand your CDROM drive and be able to boot a usable linux distro. If that is the case then you can ...
Make all the partitions on your drive you need, I always make about a 1gb /var folder your mileage may vary. if you need a / a /boot /root /whatever set them up the way you want too.
okay good. now the fun part, pick the partition you want mount it. (for my example I would have picked the /var partition) Extract the directory /slackware off the cd to that partition - okay you are in knoppix and my cd drive is occupied how do you do that - well you
could download it again from an FTP site - you could temporarily host and FTP or NFS or SMB server from another computer or use a USB CDROM - don't forget that if you have a USB hdd - you can easily copy the /slackware directory off the cd to the to the hdd - even using windows is not a problem for this task as permissions are irrelevant for the slackware installer, if you put the files on the USB hdd, you'll need to mount it on the computer running knoppix and copy it over - don't forget to format that partition the way you want too.
now that you have the other partitions define and that you have that partition formated, with the slackware folder on it. Reboot into the slackware setup CD, choose your swap partitions, and your target partitions - slackware will tell you there are more and ask you where you want them - be mindful of what partition you installed the slackware files to - as to not format it - that would mean having to do some file copying again.
Once thats done everything should be mounted in /mnt
your install files will be in /mnt/[mountpoint]/slackware
where mountpoint is whatever you chose for that partition... slackware will ask youe where is when it moves on to the "souce question"
-Hope that can be of help to others - one more tip for the "pros"
I have had trouble finding a way to get one device or another working during the setup - I realized that nothing cares about WHAT device you are installing, and any format is supported on any block device, so I quickly dd if=slack.iso of=/dev/sda (DON'T DO THIS BTW UNLESS YOU KNOW WHAT IS HAPPENING) - then I proceded to mount /dev/sda on my booted slackware setup - or even a stranger one, you can losetup /dev/loop5 slack.iso (pick a high one as I think slack uses one) - then delete /dev/hdc or hdd nodes whichever you arent using and do a ln /dev/loop5 /dev/hdc or hdd - slackware WILL detect it as a CD

- note that it will spit out some wierd errors, but yet procede to work anyway.