blackartz

OK figured this out on my own too:

Using the built in functionality of genesis_custom_field I just needed to hook it to the right part of GFWA by adding the following to the functions.php of my child theme:

add_action( 'gfwa_before_post_content', 'gfwa_grant_type_meta' );
function gfwa_grant_type_meta() {
genesis_custom_field('grant');
}

Incidentally, it works just as well if you add the Custom Meta Boxes library, which gives you a more sophisticated approach to creating custom field boxes.