Choosing FreeBSD as Startup Disk on Intel Mac 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.
It appears to be quite complicated to boot an OS other than Mac OS from USB. See this thread on Ask Ubuntu for discussion on someone trying to do similar. 

I know you don't want to install anything on the host system, but installing rEFInd or the older rEFIt may get you there. In that aforementioned thread a few people mentioned that once they had it installed a non-Mac OS USB drive would be listed as bootable from the bootloader menu.