> Is any global variable (including one that's `nil`) acceptable?
Yes, but see below.
> If you use the same mechanism, that means that `with` again allows arbitrary undefined variables, so it doesn't really improve the situation much w.r.t. accidental globals.
The outermost with-declaration can mention any name. After that, any name, in an inner with-declaration or otherwise, must either be one in the innermost with-declaration or be defined locally in the innermost with-scope. So your room for accidental globals is very, very small.
> In the simplest non-magic case, all you say is _ENV = module( ... )
See, you think this is not magic :)