I'm not too familiar with diskemu, but is it possible to integrate diskemu with the current UBCD distribution alongside memdisk? I remember people having problems with some disk images not booting under memdisk (e.g. quicktech pro).
I also can't boot quicktech pro in memdisk (with raw, bigraw, saveint, emm) or with diskemu. It boots fine when used as floppy image in a virtual machine. I think that quicktech pro overwrites a part of the memory where the driver for the floppy emulation (memdisk an diskemu) resides (not sure).
You can load diskemu as kernel in the isolinux config files.
MENU LABEL diskemu images
to download diskemu1x.
Add the necessary text in diskemu.cmd (must be in the root of the cd). If you have different images that you want to boot with diskemu, you have to add the necessary commands in diskemu.cmd.
I successfully booted the freedos image of ubcd with diskem1x (ungzip dosubcd.igz first).