One interesting thing about the SIB model is that it's being developed right now. It's shifting and changing, and a lot of the literature is just trying to get a handle on it.
One model is to contract with an intermediary, for government to contract with the intermediary. It's the intermediary that designs the project, finds the private investors, finds the service agencies, and contracts with the service agencies.
Another model could be that government works directly with the service provider or works with the non-profit. That non-profit either delivers the service or partners with other non-profits to deliver the service model. That could be one way in which the relationship between government and this non-profit would be quite a bit closer, right?
There's a question as to how these would actually operate and who that intermediary is. Also, does this prevent the non-profit from actually informing government policy or talking to government? There's that question too.