I agree that this strategy goes against the spirit of party membership, and I’m sympathetic to norm-subscription in a lot of contexts. But are norms a weightier consideration than the reasons for taking up the strategy outlined by OP? While it’s true that these vulnerabilities might eventually be closed, it might still be good to exploit them while they’re open.
To what extent do you think the relatively small numbers of EAs taking advantage of this strategy will sow mistrust? To me it doesn’t seem like it will make a lot of difference, and indeed there might be some positive signalling to be gained if people think that engaging in strategies like this is cool and smart. It might make Labour supporters look more intellectual.
(I upvoted your comment because it was an original contribution made in the spirit of curiosity, even though I doubt its suggestions.)
In my experience watching people comment on political strategies that bring terms like “hacking” to mind, most don’t see it as especially “cool”.
I’ve seen people react skeptically even to ideas like “vote pairing”, which is used to get around the oft-derided kludge of the U.S. electoral college and doesn’t necessarily harm any particular party or interest. Voting in the leadership election if one isn’t an active Labour supporter seems like an effort to dilute the values of active Labour supporters, which I don’t see as very appealing to… active Labour supporters.
I agree with Haydn that this seems like a reasonable thing to do if you actively want Labour to have more influence and you think the cost is worth it (though I don’t have an opinion on the cost/benefit model in the post), but I’m with Greg on this not seeming very ethical if you aren’t a “sincere supporter”.