Does this field type exist?

Hi Will,

You can follow this documentation to create the field post and add the setting 'multiple' => true to select multiple posts.
https://docs.metabox.io/fields/post/

'fields' => array(
    array (
        'id' => $prefix . 'post',
        'type' => 'post',
        'name' => esc_html__( 'Post', 'text-domain' ),
        'post_type' => 'page'
        'field_type' => 'select_advanced',
        'multiple' => true,
    ),
)

screenshot in MB Builder https://share.getcloudapp.com/6qu2dN68.