You do not need the hack. I just tried the 404 error code and it worked fine (the browser returned a page not found message instead of Geeklog)
Lets review what you need to do.
1. Create a new staticpage
2. Fill in the following Fields:
Title: 404 Not Found
Page Format: Blank Page
ID: the id of the staticpage you want not found.
Content: header('HTTP/1.0 404 Not Found'
;
PHP: execute PHP
3. Save the Page
One of the Geeklog Core Developers.