Today I have experienced how to install FreeBSD from a USB device - quite simple so you can do it yourself :-)

First, initialize the USB solid state disk with bsdlabel(8):

bsdlabel -wB /dev/da0

Then, initialize a file system:

newfs -O1 /dev/da0a

Third, copy CDROM’s /boot to it.

And you can now boot from the USB disk.

Similiary, you will be able to build your own tiny system this way.