Ok well if I understand you right (which I don't thing I don't) add this to your 404.shtml:
PHP Code:
<!--#include virtual="/path/to/script.php" -->
Then a make a php script (example script.php) and do this in it:
PHP Code:
<? include("/path/to/content.page"); ?>
Make sure 404 is at least 550 bytes if I remember right.