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 eugenics in a pro-eugenics direction, but they seem very willing to shift the conventional meaning of eugenics in an anti-eugenics direction whenever anyone asks.
I think probably things inside and outside the Overton Window require different strategies. "Ballsy countersignaling" might work differently for things outside the window than for things inside of it. I agree that the politician shouldn't call abortion murder.
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.