I just tried adding /bin/expr to boot-tiny.fs. I added a line to
ramdisk-tiny/list:

    LINK	ramdiskbin			bin/expr

and added expr to a line in ramdisk-tiny/ramdiskbin.conf:

   progs bad144 cat chmod chown chroot cp dd df disklabel dump ed expr

when I do make in that directory, I can see that it's doing "something"
with expr.o, etc, but when I cd over to bootfloppy-tiny, make, and
create a diskette from the resulting obj/boot-tiny.fs, it doesn't have
the expr command, and even creating a link from one of the other
commands to expr results in it printing a usage message for ramdiskbin
when I run it.

So what is the other change I'm missing?