I found the solution; it seems I was all around it. In summary, there are two methods that will work.
1) copy the austrumi iso to UBCD images directory and rename it to a simple, standard name like austrumi.iso. The mkisofs options I was using were changing the multiple periods in the original name to underscore characters. Then extract the two, large files from the iso's boot directory to the boot directory of the UBCD build. Next, add a line to one of the menu scripts (e.g., custom.scn) such as
if ($lastKey == key[f7]); then diskemu \images\austrumi.iso
This method won't work in qemu emulator because it attempts to cache the entire CD and unless you have huge amounts of ram, it will fail and send you to boot prompt at runlevel 3.
I tried this method before, but it didn't work because of the filename problem and the limitations of the emulator. Also, at one point I had the boot files in the UBCD root directory, instead of the boot directory.
2) the second method is to extract all the files from the austrumi iso, except
and place them in the identical directories of the UBCD. Then add the following line to one of the menu scripts.
if ($lastKey == key[f6]); then isolinux bzImage initrd=initrd.gz dolivecd rw init=/linuxrc root=/dev/ram0 devfs=nomount load_ramdisk=1 prompt_ramdisk=0 ramdisk_start=0 4
the string of parameters after the isolinux command are identical to those found in austumi's isolinux.cfg file except the statement cdcache is removed. As before, telling the kernel to cache the cd will take too much ram and the boot will drop to runlevel 3. Remove cdcache from the command line and the distro will boot normally even in the qemu emulator (normally I only run qemu with the parameter -m set to 192).
Of the two, the second method is the best option because it saves CD space (i.e., no need to have both a copy of the iso and it's large boot files) and it allows you to control how the kernel is started. With this method you essentially gain control of the isolinux.cfg file. Also, in the case of austrumi at least, it doesn't break other isolinux distros because the isolinux.cfg is bypassed.
Thanks again for all your help.