I've tried this today - unsuccessfully:
In SpecialRecentchanges.php, I've changed the code as you've described: 'namespace' => null to either one of the following: 'namespace' => '2', 'namespace' => 2, /* not sure if that makes a difference in PHP */ 'namespace' => NS_USER, /* probably the best solution */ (as well as changing "invert" to true)
However, nothing has changed whatsoever; the RecentChanges list is still the same (I've also tried it with the URL parameter "action=purge", but to no avail). Even if I do not invert the results (i.e. only want to display results from the USER namespace), the RecentChanges list does not change one bit.
This is very puzzling to me - though I assume there's a very simple explanation for it... !?
-- F.