Many of the builtin commands are just symlinks to busybox.
ls -l /bin /sbin /usr/bin /usr/sbin /usr/local/bin | grep busybox
/usr/local/bin/mc is just 972K
mcdiff, mcedit, mcview are the same size though, and seem to actually be just copies of mc . I wonder if those would work if they where just symlinked to mc just like with busybox.
EDIT:
Yep, that works. mc.tcz could be repacked using symlinks for mcdiff, mcedit, mcview .
However, this doesn't reduce the size of mc.tcz . I guess squashfs is smart enough to detect duplicates.