Dynamic .htaccess 404

Question

Currently, about half of our webpages derive from our HTML_page.php.

I want to add a <div class="Error404"> tag, 404 div, there to display 404 error information.

If a non-HTML_page.php webpages triggers a 404 error, I cannot display an error message using my 404 div.

UPDATE: A non-HTML_page.php would be any of our older, legacy pages that do not yet inherit from our "HTML_page.php" base class and all external websites that link to items on our domain that may not exist.

So, the default structure still needs the 404 routed to our 404.php page, but modify the routing if I can detect that the webpage contains my 404 div.

Here is how our .htaccess page starts out now:

# Custom 404 page
ErrorDocument 404 /eForms/404Form.php

#
# Turn on rewrite engine
RewriteEngine on

#
# Rewrite rule for document secuity
RewriteRule ^(.+)\.pdf$ /docgateway.php?source=$1.pdf [L,NC,QSA]

This snippet of jQuery's hasClass illustrates the logic I want:

if ($("div").hasClass("Error404")) {

Is there any way to build that type of logic into the .htaccess file for the ErrorDocument?