Is rubber-hose cryptanalysis the concept of extracting info by torture? I guess... I mean, if you're willing to beat someone to get their SSN, you could probably do a lot more harm already just using that info to apply for CCs and loans in their name.

Maybe I'm mistaken or confused here, but in that specific case you could just give any random 9-digit sequence and it would suffice? A non-SSN voter ID would work just as well for a key.

Are people threatening others based on their votes these days?

EDIT: /u/ninjanomnom brought up a good point regarding heads-of-households, which I hadn't thought of before. I suppose some sort of method would be necessary to obfuscate your vote in some situations.