I think you'll have to patch the login page for that (qa-include/pages/login.php). Check one of the other pages that already have a captcha as an example (e.g. qa-include/pages/feedback.php).
You'll probably need at least something like this:
...
require_once QA_INCLUDE_DIR . 'app/captcha.php';
$usecaptcha = qa_user_use_captcha();
...
if ($usecaptcha)
qa_captcha_validate_post($errors);
...
if ($usecaptcha && !qa_is_logged_in())
qa_set_up_captcha_field($qa_content, $qa_content['form']['fields'], @$errors);
...
Untested, so take it with a grain of salt.
Beware, though, that changes to core files will be lost when you update Q2A, so you'll have to re-apply your changes after an update.
Also, are you using anything besides reCAPTCHA so far? There are several anti-spam plugins available, and there's also Amiya Sahu's Registration Blocker plugin, which I forked recently to add some more features.
