Access page.tpl.php variables from html.tpl.php
Sounds like a perfect use case for the more generic hook_preprocess:

function MYTHEME_preprocess(&amp;variables, $hook) {
  if ($hook == 'page' || $hook == 'html') {
    $variables['shared_var'] = 'foo';
  }
}


$shared_var will be available in both html.tpl.php and page.tpl.php. This will be processed twice, so if it's expensive to build the vars, make sure you statically cache them.