> How can you load the whole module hiearchy (many modules, dependent on
> one another) with a single command in toploop?

You cannot.  However, you could build a .cma archive via a Makefile:

    ocamlc -a -o mylib.cma a.cmo b.cmo

and issue the toplevel command #load "mylib.cma", which will load both
module implementations at once.

- Xavier Leroy