I've made test system for using DOS bootdisk with USB flash drive. It uses SYSLinux so it looks like and works almost the same as ISOLinux.
To use it one should unpack attached file into bootdisk directory, and set USBFLASHSIZE variable in makefile. I've tested it only with 128M drive and I wonder whether it will work with other drive sizes. It creates usbflash.img file which should be written to USB drive.
1. If you already have bootable drive you can just replace bootsector with syslinux (you have to download it yourself) and copy contents of bootdisk/usbflash directory on USB drive.
2. it requires dc (desk calculator) to be installed.
3. Using it on windows. You should have mtools with latest patch for it to work. I've made binary Cygwin package of mtools that can be installed using Cygwin setup program. On the mirror selecting page when you are asked which Download Site you want to use, you should choose two: one for the ordinary Cygwin packages and the other for the additional packages. For the first, choose any appropriate geographically nearby site. For the second, add this User URL: http://ekot.narod.ru/unattended (The list of available download sites is a multiple selection list box, so you can <Ctrl>-click to select more than one site. The URL might be displayed in the list in an abbreviated form as just http://ekot.narod.ru even though the full URL has been accepted.
I've also put here cdrtools package and dummy unattended package which contains nothing but all dependencies required for using unattended.
To write image on drive you can use modified rawwrite program I put it here http://ekot.narod.ru/unattended/rawwritewin.zip
Description: GNU Zip compressed data