I found that they are rather fussy in their requirements.
This method uses the cheat codes and does not use partnew command.
For Fedora, the ISO file does not need to be contiguous.
For CentOS, the ISO file does not need to be contiguous if it is well below 4GiB in size so it will fit in memory. CentOS-7.0-1406-x86_64-DVD.iso is just under 4GiB but the file must be contiguous as it is too large to load into memory.
Note that because we must use a FAT32 USB drive, we cannot have files over 4GiB.