OmniBoot v1.5

A. FAT32-drive (most recommended method):

%DL ... driveletter of your USB or HDD drive
1. Format your drive:
format %DL: /fs:fat32 /v:OMNIBOOT /x /q 2. Copy CD/DVD-root to the root of your target drive.
3. Make the drive bootable with syslinux:
%DL:\boot\make\bin_win\syslinux -f -ma -d boot\syslinux %DL: FAT16 would also work, but has little importance with its 2GB-limit.

B. NTFS-drive:

Syslinux 6.03 now works better from NTFS. Please experiment at your own risk with /fs:ntfs and everything else like in A.

C. Isohybrid-Method, since OmniBoot-isos are isohybrids

Beware that this leaves the drive with a read-only filesystem with no writable empty space left!
And you need to use third-party tools like Win32 Disk Imager to write the iso to the drive.