I think I figured out how to solve the problem. I changed wrapper.tt2 this way (applying this advice):

[% IF template.naked;
    content;
   ELSE;
    content WRAPPER site_wrapper.tt2;
   END;
%]
[download]

Then, in my template:

[% META naked = 1 %]                                 
[% WRAPPER site_wrapper.tt2 %]                                        
+          
whatever                                               
[% END %]
[% FOREACH c IN categories %]
  [% FILTER redirect("${c.name}/index.html") %]
  [% PROCESS portfolio/category %]
  [% END %]
[% END %]
[download]

site_wrapper.tt2, as you can guess, contains the code that was in wrapper.tt2. In portfolio/category:

[% WRAPPER site_wrapper.tt2 %]
it works!
[% END %]
[download]

Now I have another problem: changing META informations in the processed template. But that's another issue.

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