Turns out the solution isn’t to use mod_rewrite at all, but instead to use mod_redirect. Maybe not as efficient, but simple, clean and even I can understand what’s going on:
Redirect /blogs/pt/index.xml http://xcski.com/blogs/pt/feed/
Redirect /blogs/pt/index.rdf http://xcski.com/blogs/pt/feed/
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blogs/pt/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blogs/pt/index.php
</IfModule>