My answer is Yes, you can but it has some limits.
Well, if you remember in every headerinclude templates always have these code:
<script type="text/javascript">
<!--
var cookieDomain = "{$mybb->settings['cookiedomain']}";
var cookiePath = "{$mybb->settings['cookiepath']}";
var cookiePrefix = "{$mybb->settings['cookieprefix']}";
var deleteevent_confirm = "{$lang->deleteevent_confirm}";
var removeattach_confirm = "{$lang->removeattach_confirm}";
var loading_text = '{$lang->ajax_loading}';
var saving_changes = '{$lang->saving_changes}';
var use_xmlhttprequest = "{$mybb->settings['use_xmlhttprequest']}";
var my_post_key = "{$mybb->post_code}";
var imagepath = "{$theme['imgdir']}";
// -->
</script>
You can use MyBB's variables in any template with one condition: it's must be defined in the correlative PHP file. (Of course that variable must be inside the brackets { })
For example, the correlative PHP file with headerinclude
template is global.php
.
That's it. Have fun with MyBB :)