Redeem Steam key to account with Steam OpenID like Humble Bundle?

Humble Bundle has started requiring the user to sign in through Steam's OpenID authentication where it then redeems the Steam key on the user's account. I was unable to find any documentation on this subject.

Is the system open for redeeming games on connected accounts to other developers to use, other than Humble Bundle? If so, how can this be done? I'm looking for example code or a link to documentation.

Solution

Steam has removed this functionality. As a result, Humble Bundle doesn't support this anymore either.