The private mailing list thing seems like it would be problematic. Are you thinking of a single private mailing list, or one for each embeded developer?
If it is just a single mailing list, then the developer's competitors will likely also be on the list, which they might consider just as bad as a public list.
If it is separate lists, that is a lot of effort for the kernel developers. Also, what should they do if two embedded developers propose interfaces that achieve similar or identical aims? Do they break confidentiality and try to get the two to cooperate, or do they have to pretend that they don't know about the other use case?