There is a trick that I'm using, set raid mode in bios, that's basically another name for ahci since the access is done through ahci and then you can boot from the odd port
Tried that once .. installed OMV onto the ODD including everything, added 4 HDD's to the device on RAID .. and than the device wanted to boot from the RAID part and not the ODD part ..
So I stick with the USB grub / ODD function. It works and if something works: keep your hands off it