I also have not put FreeBSD on an iMac but did look into installing linux/BSD into an iBook that was given to a friend with the hard drive removed (No it was not stolen - the owner had enough money to buy a newer iBook and pulled the hd to put into his new purchase). We thought about picking up a refurbished hard drive to generate a working laptop.
What I recall is that the Apple BIOS was configured in such a way that OS/X had to be installed and a chain loader could be added to boot a second operating system. I would advise you to search the
Freebsd forums for dual booting with OS X and the internet. I think one tool you will need is called refit.