I'd think it has to be rc or rc.asus (but I couldn't find it either), both of which are untouchable in 1.0.4.6.
Perhaps a better long-term approach would be to bolt on an upgraded busybox via a boot-up script. By putting the new busybox first in the search path, any conflicting Asus busybox functions will be superseeded.
That way when Asus releases their next version of firmware, there's less work to do for us to get it running the way we like.
- K.C.