i want that registered user can see DIFF button and everything, and non-registered can show only 'recent changes', so my wish is to have a DISABLEACTION function managed by a sort of ACL.

A simple way to solve this problew is it that follow.

in /incl/template.php, change:
case 'diff':
 html_diff();
    break;

with:

 case 'diff':
    if ($_SERVER['REMOTE_USER'])
    {
        $first = is_numeric($_REQUEST['first']) ? intval($_REQUEST['first']) : 0;
        html_diff($first);
    }
    else
        html_login();
    break;

in this way, if you are a logged-in user, you can display the DIFFERENCE section, with code confront.
if you are not logged, it displays a form for login.

This document describes the steps needed to integrate DokuWiki with Mantis. The method described here was originally tested with mantis version 1.1.0rc2 and dokuwiki rc2009-12-02, and should work with subsequent versions (modulo bugs). It may work with older versions of mantis provided you make the File Changes described below.

template Simple and lightweight, but modernized version of the default template

Dokuwiki white template installation