...and the debate about whether Plan Y is or is not a good idea would immediately shift to a debate about whether Plan Y is or is not eugenics - which you would lose, because it is.
This assumes words have true, immutable meanings, which they don't.
For example, if you respond to the claim that Obama is a communist by...
Wait a second, you yourself explained this pretty well in your Anti-Reactionary FAQ: "The meaning of words changes over time, and the Cold War made the more moderate elements of communism drop the 'communist' label."
Accusations of "eugenics" generally deserve a similar response: meanings of words change over time, and the fact that a policy fits Galton's original definition of "eugenics" doesn't mean it's "really" eugenics any more than we should examine pre-Cold War communist documents to establish whether America is "really" a communist country.
Yes, arguing about definitions is annoying, but there's really no way around having to explain that no, policy Y is not what people commonly associate with "eugenics." The real choice you have is whether to alienate a portion of your audience from the outset by declaring you like eugenics.
And yes, ballsy countersignalling can sometimes work, but this doesn't mean it's automatically a good idea in every case. For example, I predict that politician who described her support for legal abortion by saying "I support murder in some cases" (*cough cough*) would have a very difficult time getting elected.
I don't think it assumes words have immutable meanings, just that they have some conventional meaning. You are proposing that we turn the debate from "Is eugenics plan x a good idea?" to "Does plan x, which fits the current conventional meaning of eugenics, sound like eugenics to you?" Unless you can unilaterally change the conventional meaning of eugenics, then for your purposes the meaning might as well be immutable - your argument will fail. And not only do people show no signs of being willing to shift the conventional meaning of eu...
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.