Surfing on the idea suggested by rx2, here it is my (so far working) solution.
1) I wrote the snippet "checkLogin" to chek login status:
$user = $modx->getLoginUserName();
$logged;
if ($user) {
$logged = 1;
}
else{
$logged = 0;
}
return $logged;
2) Then, in the chunk where I want to display content login related, I check the login status through MODx output modifiers applied to "checkLogin" snippet (here, for instance, I show a link to add a new post to the blog only if the user is logged in):
[[!checkLogin:is=`1`:then=`<a href="[[~245]]">Write new post</a>`:else=``]]
Or, if you prefer, you may also test it through a call to the "If" snippet:
[[!If?
&subject=`[[!checkLogin]]`
&operator=`EQ`
&operand=`1`
&then=`<a href="[[~245]]">Write new post</a>`
&else=``
]]
3) Where "[[~245]]" is the page containing the "NewsPublisher" snippet, properly parametrized:
[[!NewsPublisher?
&show=`pagetitle,published,introtext,content`
&initrte=`1`
&initdatepicker=`1`
&rtcontent=`1`
&rtsummary=`1`
&parentid=`67`
&template=`12`
&classkey=`Article`
&hidemenu=`1`
]]