It's not all or nothing. The more inclusive the enfranchisement, the more cooperation there will be in general.

That's not clear to me.

Suppose the Blues and the Greens are political opponents. If I credibly commit to pointing my CEV-extractor at all the Blues, I gain the support of most Blues and the opposition of most Greens. If I say "at all Blues and Greens" instead, I gain the support of some of the Greens, but I lose the support of some of the Blues, who won't want any part of a utopia patterned even partially on hateful Green ideologies.

This is almost undoubtedly foolish of the Blues, but I nevertheless expect it. As you say, people aren't all that smart.

The question is, is the support I gain from the Greens by including them worth the support I lose from the Blues by including the Greens? Of course it depends. That said, the strong support of a sufficiently powerful small group is often more valuable than the weak support of a more powerful larger group, so I'm not nearly as convinced as you sound that saying "we'll incorporate the values of both you and your hated enemies!" will get more net support than picking a side and saying "we'll incorporate your values and not those of your hated enemies."

With that scheme, you're incentivizing folks to prove they have enough political clout to get in your way.

Sure, that's true.
Heck, they don't have to prove it; if they give me enough evidence to consider it plausible, I'll include 'em.
So what?

Moreover, humans aren't perfect reasoning systems. Your way of determining enfranchisement sounds a lot more adversarial than mine, which would affect the tone of the effort in a big and undesirable way.

I think you underestimate how threatening egalitarianism sounds to a lot of people, many of whom have a lot of power. Cf including those hateful Greens, above. That said, I suspect there's probably ways to spin your "include everyone" idea in such a way that even the egalitarianism-haters will not oppose it too strongly. But I also suspect there's ways to spin my "don't include everyone" idea in such a way that even the egalitarianism-lovers will not oppose it too strongly.

Why do you think that the right to vote in democratic countries is as clearly determined as it is?

Because many people believe it represents power. That's also why it's not significantly more clearly determined. It's also why that right is not universal.

Restricting voting rights to those of a certain IQ or higher would be a politically unfeasible PR nightmare.

Sure, I agree. Nor would I recommend announcing that we're restricting the advisory board to people of a certain IQ or higher, for analogous reasons. (Also it would be a silly thing to do, but that's beside the point, we're talking about sales and not implementation here.) I'm not sure why you bring it up. I also wouldn't recommend (in my country) announcing restrictions based on skin color, income, religious affiliation, or a wide variety of other things.

On the other hand, in my country, we successfully exclude people below a certain age from voting, and I correspondingly expect announcing restrictions based on age to not be too big a deal. Mostly this is because young people have minimal political clout. (And as you say, this incentivizes young people to prove they have political clout, and sometimes they even try, but mostly nobody cares because in fact they don't.)

Conversely, extending voting rights to everyone regardless of age would be a politically unfeasible PR nightmare, and I would not recommend announcing that we're including everyone regardless of age (which I assume you would recommend, since 2-year-olds are human beings by many people's bright line test), for similar reasons.

(Somewhat tangentially: extending CEV inclusion, or voting rights, to everyone regardless of age would force us as a matter of logic to either establish a cutoff at birth or not establish a cutoff at birth. Either way we'd then have stepped in the pile of cow manure that is U.S. abortion politics, where the only winning move is not to play. What counts as a human being simply isn't as politically uncontroversial a question as you're making it sound.)

Again, this is a different argument about why people cooperate instead of defect.

Sorry, you've lost me. Can you clarify what the different arguments you refer to here are, and why the difference between them matters in this context?

Ultimately it probably doesn't matter much what their broadcasted intention towards the enfranchisement of those outside their group is, since things will largely come down to what their actual intentions are.

Once they succeed in building a CEV-extractor and a CEV-implementor, then yeah, their broadcast intentions probably don't matter much. Until then, they can matter a lot.