AMI BIOS boot options?

I've not specifically looked at the BIOS options on a 312 (don't have one) or product in the same family, but that seems to allow either a UEFI or legacy BIOS boot-up from the same source just as many other AMI BIOS do.  With most BIOS, it will check all USB ports for a bootable drive since it doesn't allow you to choose which one.  I think your best bet is to simply try it and see what happens, then let us all know in case we need to follow your lead.  One potential problem is that the flash memory on at least most ReadyNAS is actually a USB device.  So if it finds that one first, it's going to boot from it.  On a legacy NAS, I know it will find the front USB port before the internal one, so try that first.

Unless you go in and make changes yourself, there is no reason that booting to an alternative OS will change anything in the boot PROM.  But your new OS may expose it to change, so you'll just have to be careful not to.  Once exposed, you will probably want to make a copy of the contents with dd (if using a Linux alternative) or some other drive imaging process just in case.