So far there's only one solution: "this can be done (for common lisp) by handling undefined-function condition and invoking appropriate restart (which is unfortunately implementation-specific)."

Which makes it sound like there is in fact no way to do this in standard CL, to say nothing of other lisps.

Untested perl code to do this (taking a cons to be a [car, cdr] array reference):

    AUTOLOAD {
        unless ($AUTOLOAD =~ m/::c([ad]{1,})r$/) {
            die "Attempted to call undefined function $AUTOLOAD";
        }
        my $ads = $1;
        my $cons = shift;
        
        while (my $c = chop $ads) {
            $cons = $cons->[$c eq 'a' ? 0 : 1];
        }
        return $cons;
    }

Example of a situation this feature might be useful: "I have this object with lots of properties which I don't necessarily know what they are. How do I define getter/setter methods on them?"