ultimately want to have a library of BIOS Iso disks (bootdisk.img) booting from my live Knoppix 7.4.2 disk frp, a boot choice menu.lst

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What you suggest may be a worthwhile endeavour; there may already ne internet guides. I expect nobody will write you one here.

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I've never configured a USB flash drive for multiboot, but something like this should work with a flash drive as well as it works with a hard drive

Your post is too vague to get any specific help. You don't indicate which distributions you want to start with nor whether you want to use a DVD or flash drive. The process is different. You can use isolinux/syslinux, Grub Legacy or Grub2 to do it. In each case, you will need to manually create the boot menu file. Booting with isolinux/syslinux will mean extracting the iso by loop mounting and copying the files to another location so they can be modified. Grub2 will allow you to directly boot an iso as explained in the post above. This won't work on 'any' Linux but will on Ubuntu and many of its derivatives and possibly others. The file that would need to be used is the grub.cfg file.

I put 10 Linux Live distributions on a DVD a few years ago booting it with Grub Legacy. Limited by the size of the DVD obviously. I have a 16GB flash drive with 20 Linux Live CDs on it booting with Grub Legacy and chainloading to a second partition with Grub2. It can be done but, you will need to post a little more info on what you want. Have you done much research as I found a number of sites with information on this.

I tend to use grub for multibooting but there are a few choices and some may or may not work with all types of boot images. Grub 2 is pretty easy and works for many. Sysliunx and isolinux may be needed to be chain loaded too.