TBH, a closed network might work quite well in promotion of the ethos - at least initially. If you recall, this is essentially the way that gmail was established.

The general idea is you and all your trustworthy mates form a network - this initially excludes everyone (hopefully) who has been a in the past - and gives the people who have behaved ethically a head start (the rewards of openscience that will come to all). I would assume these people would recruit as many trustworthy people as they could, and soon all the community is more or less involved. Those who had previously survived by knowingly prepublishing or other unethical practices might see that they have lost an advantage and reform, thus being invited by colleagues. So a peer-pressure mechanism at work to enforce ethics, as there will be an advantage to being part of the group. And there isn't any reason to stop interested parties who are not practicing scientists joining.

And then no reason not to open out once there is an established network of trust and enough people to keep it going.