I used the same principle to utilize the wordpress custom meta fields to get the same result. So I have a two fields named productname and productprice that I can enter values into and it works great.
<?php $product_id = get_post_meta($post->ID, ‘productname’, true);
$product_price = get_post_meta($post->ID, ‘productprice’, true);
echo echo print_wp_cart_button_for_product($product_id, $product_price); ?>
My question now is how do I get variations, like for tshirt size?