[watwut]

"The generated password is also site-unique and thus leaves you more resilient against sites losing their password databases or being outright malicious"

Assuming this tool would become popular, I do not think it would make any difference in scenario you described. If I know that a lot of people use this to generate their passwords, I can:

* guess the door id (e.g. facebook or fb for facebook.com),

* concatenate it to usual attack guess,

* hash the result one more time and continue in exactly the same way as usually.

Bonus for attacker: if any of password databases leaks and attacker manages to acquire your passphrase the above way, he needs only few door id guesses to get access on any of yours accounts.