I needed the option to set a different message for each Node Checkout node. For instance, purchasing an event would have a message saying "This event requires registration information." or a custom product might need to say "This t-shirt requires some additional order details."
This patch adds the option to set a custom message per-node, as desired. I think everything should be per the Drupal best practices, but if I've done something particularly goofy in my code, feel free to let me know and I'll address it. Thanks!