>> Of course one way to do that is if the format has \let\def\@undefined

How can that work?  Assuming that (e.g.) "\newcommand" is
macro -> {... \def ...}, then without a LaTeX equivalent of PostScript's "bind",
you still need access to "\def"; and if "\newcommand" is
macro -> {... \someconcealedversionofdef ...}, then the user can
use "\someconcealedversionofdef" :-(

** Phil..