Last question first: Yes you can runZipSlack from a USB stick. Just unzip and then do syslinux /dev/sda. If you're USB isn't bootable fromn the BIOS then you can use the boot floppy for RUNT Linux to boot it. However, the life of your USB stick will be quite short as the number of read/writes is limited with FLASH. If you turn off system logging and setup a swapfile on another device it will help.
Now about loadlin. I'm not sure that it will run from a DOS prompt under NT/XP. I you can start the machine in DOS mode it should work though. You could always put loadlin on a DOS bootable floppy and do it that way.
You may need to change your config.sys and put this in there:
dos=umb
devicehigh=himem.sys testmem
ff
devicehigh=emm386.exe noems I=B000-B7FF (B and 3 zeros)
I run ZipSlack quite happily with a Win98 system. You might want to read my HOWTO at:
http://home.earthlink.net/~gnashley/...ix8_1howto.htm
Let me know if this works for you.