Since this issue doesn't seem like a priority and i am in need of this feature, i would like to propose a possible solution:

Using php's $GLOBALS you could set those values in say $GLOBALS['PREAMBLE']['SERVER'] inside the preamble script and have PhabricatorStartup::normalizeInput merge them into $_SERVER after filtering happened.
This obviously means that the preamble script has to be configured using $GLOBALS, instead of setting $_SERVER directly.

Example (inside preamble.php):

<?php

$GLOBALS['PREAMBLE'] = array (
  'SERVER' => array (
    'REMOTE_ADDR' => $_SERVER['HTTP_X_FORWARDED_FOR'],
    'HTTPS' => true,
  ),
);

I tested it on my phabricator instance and it seemed to have worked, though me not being a php programmer might mean that i overlooked some things. ;)

Will attach a diff for review!