I partitioned a USB flash drive as GUID and used it to install FreeBSD 10 for x86_64 off a DVD on a MacPro1,1. I was able to select the installation DVD as my startup disk in Mac OS X just by going to System Preferences > Startup Disk and selecting it. But OS X (I tried SL, ML, and Mavericks) can't read FreeBSD startup partition on the flash drive because it's formatted as FreeBSD_UFS. As a result, I can't select the FreeBSD partition as my startup disk.
I also tried this:
sudo bless --device /dev/disk4s2 --setBoot --legacy followed by a restart (disk4s2 is the FreeBSD partition of course). It just booted back into OS X.
I tried holding the option key at startup. The FreeBSD partition was not an option. On a PPC Mac, it is an option, but of course I need the PPC version of FreeBSD to actually boot from it. On a PC with a BIOS, I can easily set the boot device to the flash drive in the BIOS settings. Unfortunately, it's not that simple on a Mac to change EFI settings.
I'd like a solution that doesn't involve permanent modification of the host machine. Maybe I can install a boot manager that can boot into FreeBSD that OS X will recognize as a startup disk on another partition on the flash drive. I don't know the best way to do that.