I think there may be a solution igadoter;
PartedMagic-4.3 is Slack-12x based, and the pmagic-hdd-4.3.tar.bz2 is basically
a full install of pmagic to usb or hdd.
Now, they get around the problem with finding the partition/usb device with a small initrd, qouted from pmagic site-
Note: The small initrd is needed to "find" the partition with the installed Parted Magic. Device numbers change from system to system and this eliminates the need to keep changing the device number in the extlinux.conf. So, in theory it should boot from a USB flash without the device numbers confusing the boot loader or computer device configuration. This does in fact run like any other distro that boots from a hard disk install. Everything you do is saved and everything you break will be there the next boot.
It does work, the extracted pmagic-hdd has a linuxarc and a kernel/initrd.
The linuxarc is a link to /bin/busybox
It found both my hd and my ext3 usb I booted it off of.
You can also add slack pkg's to pmagic
Parted Magic is not exactly Slackware based, but the main tool chain was compiled on Slackware. If you would like to add programs your best bet is to use TGZ packages from Slackware 12.x. You can also use DEB files, but Parted Magic doesn't make any attempt to resolve dependencies. Copy the packages to /pmagic/pmodules and Parted Magic installs the packages automatically at boot. If you are using the USB version the packages can be simply copied to /pmagic/pmodules. If you are using the LiveCD version you must remaster the iso file with the add packages. As of Parted Magic 4.x there is a spot in the "save session" routine that allows you to add packages to the CD that you used to boot Parted Magic.
so, there must be a way to somehow hack slack and pmagic
and maybe the initrd will help autofind usb, as with pmagic?
There is a extlinux.conf in boot/extlinux that has the kernel/initrd
convert that into menu.lst if using grub, like me.
So, I think you could "Macgyver" this method into slack usb
and it should work...right?
I think this is good info and does relate to what you wanna achieve.