Choosing FreeBSD as Startup Disk on Intel Mac
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.