It's not something I have done either, but there are a couple of ways. One is to install to the USB key, as ua549 suggested. A simpler way would be to use
boot0cfg(8) to add the FreeBSD boot manager to the USB key. That boot manager would then allow you to select the FreeBSD slice to boot from. Search for boot0cfg on the forum for another few threads where we have recently discussed this utility. Also read the man page linked earlier in this post for more information.
Edit: OK, I just read your post properly. You don't need the /boot folder - you should have the root partition on the hard drive. Then when the USB key brings up the boot manager, select the freeBSD partition. Unless you put the full root partition on the USB drive, attempting to boot from it will not work.
(Re: my sig - OK, there is a dumb answer given!)