This would be really useful. I've had 2 people so far sign up to my new site who have been put into moderation. Both of them a people I personally know, and ar registered on my other XF site, so it would have been good to see exactly what triggered the moderation.
From what I can see in the ACP, it doesn't log anything. I had a spammer try to sign up on my dev site as it's not behind a .htaccess at the minute while I test some things out, and the option set is manually approve when at least 1 of the warning flags are triggered. There was no log of this sign up, and as I rejected it, that doesn't appear to be logged any where either.