[development] postprocess theme in Drupal 6?

[Dave Cohen]

I'd like to override theme_page() with the following logic:  first use the 
page.tpl.php template, as the theme normally would; then, do some special 
processing to the output of the template.  In drupal 5 this was easy enough, 
in drupal 6 I can't figure out how.  If you can help, please respond to 
http://drupal.org/node/262085.

(Sorry to bother the devel list with what might be considered a support 
question, but I'm stumped.)