Giving unfiltered_html Capability to Site Admin?

Anyone have an idea how I might go about this? I thought I could use the regular role manager process but site_admin is not a role that capabilities can be added onto.

I know I could do something like:
if (is_site_admin())

but is there any way to use the actual unfiltered_html role? Maybe something at initialization on each page load that would temporarily give site_admin that capability (but not save in db). The code in the Unfiltered MU plugin seems to do that:

function um_unfilter_roles_one_time() {
get_role( 'administrator' );

global $wp_roles, $current_user;

$use_db = $wp_roles->use_db;
$wp_roles->use_db = false; // Don't store in db. Just do a one off mod to the role.
um_unfilter_roles(); // Add caps for this page load only: - ^^^^^^^
$wp_roles->use_db = $use_db;

if ( is_user_logged_in() ) // Re-prime the current user's caps
add_action( 'init', 'um_unfilter_roles_one_time', 1 );

Can I modify this function for just Site Admin?