AFAIK, HN has no OAuth2 or similar, so a SAAS would not work for this task; the users would have to host such software themselves on someplace that they trust with their HN password.

I can tell you are a very creative, intelligent, and resourceful person and am confident you have better uses for your time :)

If you tell me your password and I downvote everyone you hate, why is that less of a service than it would be if, instead of telling me your password, you gave me a special password with fewer permissions?

The first service comes at the cost of losing one's entire account. The second one does not. Therefore the second one is far more valuable.