Booting a floppy image on a flash drive
The easiest way is to start with an already working Ubuntu installer disk.
First you need a copy of "memdisk" which is part of the debian "syslinux" package, generally found in/usr/lib/syslinux/memdisk; copy it to /install/memdisk on the flash media. Also copy the bootable floppy image into the root of the flash media. Many such images can be found at bootdisk.com. Then edit /syslinux/txt.cfg and insert a menu item for your floppy image:
label floppyimage menu label ^Boot floppy image kernel /install/memdisk append initrd=/floppy.img
Now when you boot a machine off the flash drive, you'll have the option of booting from that floppy image.