Can .htaccess test for If-Modified-Since?

Yes, of course

RewriteEngine on
# check
RewriteCond %{HTTP:If-Modified-Since} ^(.+)
# and pass as query strng q=
RewriteRule !^304\.php$ /304.php?q=%1 [L]

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caterham_www, I think you clearly understand what I am trying to do.

If a server has already sent a file or an image I do NOT want to send it again.  Instead I want to tell them to stop wasting time and bandwidth by sending them a 304 header.
https://www.experts-exchange.com/questions/24168325/Send-304-Header-if-cached-Apache.html

Will this work the way I intend it to?  Will it save website visitors time and my server bandwidth?

After giving my last question some thought, do you think it will work about as good as your ideas to modify and  recompiling the mod_rewrite.c module or make a custom Apache module?

<?php
   // 304.php
   header('HTTP/1.1 304 Not Modified');
   exit();
?>

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