How can I make special non-Drupal URLs without file extensions?
If the file exists serve it. 

  RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI}\.html -s
  RewriteRule .* %{REQUEST_URI}\.html [L,T=text/html]


You'll need to put it above this inside your .htaccess file.

  # Pass all requests not referring directly to files in the filesystem to
  # index.php. Clean URLs are handled in drupal_environment_initialize().
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteCond %{REQUEST_URI} !=/favicon.ico
  RewriteRule ^ index.php [L]


You can't pass the request back to apache once you've passed it to Drupal. The code above will hit the .html file first if it exists.

To do exactly what you want, you can set the 404 handler inside of drupal to a custom page; that page could then check to see if the file exists and use that. Config page for 404: admin/config/system/site-information. The handler would use something like this 

&lt;?php
$path = substr(request_uri(), strlen($GLOBALS['base_path'])) . '.html';
if (file_exists($path)) {
  echo file_get_contents($path);
  exit;
}
return MENU_NOT_FOUND;
// OR this, not sure.
return drupal_not_found();
?&gt;