adding meta data using plugin to top of head

Okay, here’s the answer:

function CustomHead() {
include('headcontent.php');
}
add_action('wp_head','CustomHead',1,1);

The key is that first “1”, which sets the priority of it to run at the highest (this is an optional element, defaulting to 10, putting it at the end of the head by default). More info here: http://codex.wordpress.org/Function_Reference/add_action