There isn't yet any API for introspecting ASTs ("parsing of passed-in code" as you call it), nor for programmatically constructing new AST nodes. Such things will hopefully emerge once macro implementation and specification become more concrete.

So I can only guess what it would look like; take the following with a rock of salt:

    macro do-in-reverse($code) {
         $code.clone(statements => $code.statements.reverse);
    }

    do-in-reverse {
        say 'First';
        say 'Second';
        say 'Third';
    };

But it might turn out not be that simple.

The diffulcty comes from the fact that unlike Lisp, in Perl 6 not everything is a list, so you can't treat the AST as a list (... of lists, possibly) either. The .clone would take care to preserve things like outer lexicals and the signature of the block (you don't see it; it's implicit) and other non-listy things.