How to set boot device when it doesn't show in boot screen I'm using my Mac Pro as a FreeNAS server. FreeNAS is based on FreeBSD, so I assumed it would play nice with Mac Pro, but it really doesn't as it relates to boot devices.

FreeNAS will only boot IF you trick Boot Camp Assistant to think FreeNAS is Windows and when it prompts you to install Windows, you switch the disk on boot-up and install FreeNAS instead.

Lastly, FreeNAS will only boot if you have a SATA drive connected to one of the 6 built-in SATA channels on the Mac Pro. It will NOT boot if you have a PCIe card with a SATA SSD mounted on it.

If you do everything right and get FreeNAS to boot from one of the built-in SATA channels, if you hold down Option while booting, no boot disks are shown. 

My concern is although Mac Pro is happy to boot into FreeNAS today, if it stops booting for some reason, how do I tell Mac Pro to boot from a disk that doesn't show up in boot drive screen?

(Note: FreeNAS has the option to mirror boot drives, so if one fails, in a normal PC hardware situation you can just point to the mirrored drive for boot. I'd like to use this technique as well for my Mac Pro FreeNAS, but since Mac doesn't recognize FreeNAS as a bootable device unless you trick it via Boot Camp Assistant, I don't see this as a viable option, unless someone knows how do it otherwise.)
You may want to try a different boot loader than BootCamp. Personally I've used http://refit.sourceforge.net/screen.html and http://www.rodsbooks.com/refind/ (a fork of refit) to boot various linux flavors on my old Macbook Pro. They're relatively simple to setup (for a boot loader).