Generally speaking, you can't copy the cross compiled files to a USB stick and use it on MIPS, at least not for the RouterBoard (MIPS) case which are all big endian (while your i386/amd64 is little endian).
So the boot strap process usually goes thru netbooting the board and then populate your SD/USB stick from it.
You can also use the makefs(8) to create a big endian file system from a little endian host, but it has some limitations as its primary use is to build RO (or very simple) images - eg. you have to manually specify the number of free inodes on file system and so on.
All the MIPS RB are quite well supported, the only show stopper right now is the missing NAND drivers (as the FreeBSD NAND Framework is alredy in tree). This is kind of my fault, since i've these drivers for some time time but never found the time to finish, clean and push it to -head. I hope i can get over it soon =)
The MIPS port have evolved a lot recently and i don't think it is really a tier 3 arch anymore... The port is stable (yes, i'm a daily user of it) and reliable.
At this moment, AFAIK, there is only a limited support for PPC RBs, you can netboot some of then, but no NAND, no CF and (not sure about this, but looks like) no pci/pcie.
Code:
rspro-01# uptime
9:10AM up 25 days, 5:05, 1 user, load averages: 0.08, 0.02, 0.01
rspro-01# uname -a
FreeBSD rspro-01 9.0-CURRENT FreeBSD 9.0-CURRENT #135 r223874+2c57285-dirty: Thu Jul 14 16:46:47 BRT 2011
root@devel.network.int.br:/data/git/mips-gpio/obj/mips.mipseb/data/git/mips-gpio/freebsd/sys/RSPRO mips
As you can see i'm using a quite old -head on a RSPRO which is very close to the RB450G (the RB has more memory).