Display CAPTCHA after form submit

In Mollom is feature — show CAPTCHA field only after form submit, i.e.:

1. user filled form (without captcha) and submit form
2. drupal show intermediate page with captcha
3. user filled captcha and submit this form

I would like to this feature in CAPTCHA module. This could solve the problem with page caching (#602226: CAPTCHA and page caching).