I was going to test
Just to save you some time: Using nand-sata-install to transfer the rootfs to a HDD (with bootloader having to remain either on SD card or eMMC) still doesn't work (at least not the combinations I tried, most probably related to OMV rootfs now relying on btrfs and not ext4 any more). Since I consider this somewhat stupid anyway (since rootfs on a HDD will prevent it to spin-down for longer periods and that's bad for these types of OMV installations) I won't look into it the next months (Armbian on Stretch has higher priority).
But transferring the installation from SD card to eMMC should work (tested myself after I had implemented a fix @'chymian' reported a while ago that I've had overlooked) while it's still a weird idea. The eMMC modules from Hardkernel are both amazingly fast and expensive and for the OMV use case there's zero benefit running off eMMC anyway. Good genuine Samsung EVO/EVO+ SD cards with 16-64 GB are less expensive and OMV 'performance' is exactly the same as long as the flashmemory plugin is active (default).