Just wanted to update that against all odds (with me being not that bright) I managed to get this to work basically on the first try!
The only real snag I hit is I already had three primary partitions on the drive so what I had to do was shrink the largest one with the most free space down to give me a 15gb unallocated partition. Then I set that up as an extended partition, creating two new partitions inside that for the SYSTEM and STORAGE partitions that EL needs.
After that I rebooted into Antix and had to use gksudo gnome-disks to allow me to mount them and copy the files into each directory from my USB drive. Then using Grub Customizer I copied the entry for my USB drive and edited it to reflect the new location on Disk 0, Partition 5 and save.
Rebooted and it showed up on the Grub and booted up exactly how I hoped.
So, yay me.