I have all of /usr on an SD card.
There is no problem as long as you relink a few /usr/bin busybox apps to /bin ([, tail, cut, test and dc) and make sure all the busybox apps in /usr/bin and /usr/sbin point to /bin/busybox or just copy busybox to where the links can find it.
I found the answer on the sourceforge OZ mailing list.
Have fun!