MT could have a key that you entered. With that key, it would take the blog ID entry, and your key, and encrypt the form fields and form actions for each blog entry so no two comment forms would be alike. No comment spam bot in hell would be able to figure that one out, and MT would carry on without issue.

The only thing a comment spam bot could do is scrape the page with the form elements, and assume it was a comments form, and try posting data to each element anyhow, hoping for the best.