a priori I'd have expected EN to be less likely than average to go on a downvoting rampage; my mental model of EN simply argues with people it disagrees with, rather than pulling some cloak-&-dagger shit

I agree strongly here. That's part of why my initial estimate was low. My initial estimate was based on the fact that there were around 5-6 users with enough karma and the apparent political motivation, and then I reduced that percentage for Eugine because he seemed unlikely to be the sort of person who would do something like this.

I've disagreed with EN before, and I'm fairly sure EN's downvoted me at least once, but I don't remember ever being block downvoted

I've been block downvoted before, also on gender/sex issues, but I'm fairly confident that wasn't Eugine. On the other hand, I'm also fairly sure base on some things that I've seen that Eugine has downvoted people while he replies to them as part of an ongoing disagreement(Edit:And he seems to be doing just that to me right now in another conversation, which is sort of amusing at some level.) And this sort of thing seems indicative of the sort of attitude that would be more likely to go and engage in block downvoting. But even given that I agree it is out of character.

shminux "would be quite surprised if whoever karma-stalked me was pissed off at anything I said about gender issues"

Right. Trying to explain all of this with one hypothetical super downvoter may be a problem. In shminux's case, he's got a lot of different controversial opinions that could potentially trigger something. The same applies to Imm's comment.

might be wrong (I don't use that feature) but I think that only makes votes on top-level posts public. (Though that information would still be suggestive.)

Hmm, in that case, this would be close to completely useless- all of ialdabaoth's submitted posts have multiple downvotes, so one could legitimately have downvoted almost all of them. The only that might be particularly interesting is this one which has only 2 downvotes.