Turns out not linking my a.out was a bad idea; I fixed it by doing this:
exit = 1.
mount = 21.
sys mount; rk0+040014; usr+040014
sys exit
rk0: </dev/rk0\0>
usr: </usr\0>
This seems to work. I think the rk fs image is not right, but Warren seems
to have a grip on that and I'll try his latest mkfs next.
-brad