I found this post very disturbing, so I thought for a bit about why. It reads very much like some kind of SF dystopia, and indeed if it were necessary to agree to this lottery to be part of the hypothetical rationalist community/country, then I wouldn't wish to be a part of it. One of my core values is liberty - that means the ability of each individual to make his or her own decisions and live his or her life accordingly (so long as it's not impeding anyone else's right to do the same). No government should have the right to compel its citizens to become soldiers, and that's what it would become, after the first generation, unless you're going to choose to exile anyone who reaches adulthood there and then opts out.

Offering financial incentives for becoming a soldier, as has already been discussed in the comments, seems a fairer idea. Consider also that the more objectively evil the Evil Barbarians are, the more people will independently decide that fighting is the better decision. If not enough people support your war, maybe that in itself is a sign that it's not a good idea. If most of the rationalists would rather lose than fight, that tells you something.

It's quite difficult to know the right tone of response to take here - the Evil Barbarians are obviously pure thought-experiment, but presumably most of us would view a rationalist country as a good thing. Not if it made decisions like this, though. Sacrificing the individual for the collective isn't always irrational, but it needs to be the individual who makes that choice based on his or her own values, not due to some perceived social contact. Otherwise you might as well be sacrificed to make more paperclips.

If it was intended as pure metaphor, it's a disquieting one.