Do you have any experience with involving people not familiar with SE community inside a proposal?
Not really, but I have some thoughts based on the reactions of people I talked with about my participation on SE communities, my own brother's attempts at participating on SE (spoiler: he's inactive now) and based on some things I noticed during my own participation on SE. I hope they may be helpful:
Showing is better than telling, so if you want someone to participate on your proposal, show them how to follow a proposal, how to create an example question, how voting is needed to pass the proposal. Rinse and repeat in private beta, and perhaps even for public beta. Have people pass the information on. That's how I got my brother started: I showed him the site and what I did on it.
Keeping people engaged is going to be much harder though. As I said, my brother is inactive now, after asking a few questions on different (not computer related) parts of SE. When I asked him why he said he had no more questions, not ones that would fit the SE system, and doesn't feel his areas of expertise include a topic where he can answer questions.
I've tried to get other people involved in answering stuff, especially if I know about sites that match their profession or expertise. The usual answer is something along the lines of 'I have other/better hobbies'.
So, the short list of involving people includes three things that I have noticed:
- Show people how SE works, don't just tell them how awesome it is.
- Make sure there is a need for a place to ask questions as a way of getting/sharing information. This need can be made up with enough programmers with an interest in Coffee or Interpersonal Skills or a bunch of experts that are currently struggling to easily exchange information. But there needs to be a need for a place to ask questions!
- Make sure the questions your expert community is struggling with will fit within the system of SE. An expert in their field of cultural heritage conversation isn't very likely to need to ask about whether they can clean The Migdale Hoard by dumping it in Coca Cola for the night. In fact, their questions may very well be more like actual research questions, which might need more than the SE system can offer. Similarly, questions that can be answered with a link to an article found through Google Scholar might not interest your expert much, making him think there are better ways to spend his free time.
Building on those points, and looking at your proposal and activity, I think you should ask yourself: Do these people really need a place to ask questions? Based on a bit of another question of you related to this proposal:
There are many journals devoted to this research, conferences and also some shows that give you an insight on the multidisciplinarity of this field. So this is a professional site which should be different from Art and Crafts. At the moment in the definition phase, unfortunately, the process of involving other people that are not familiar to SE community it's hard because they follow the group but then they can't understand how to make an account to post and vote answer. So I am waiting to advertise the site to other older restorers that might give up if the process is too complex.
To me, that sounds like there is not much need within your expert community to have a place where they can ask questions and get answers, they have already found other ways to do that. And I'm sorry to be a pessimist and say that any attempt at getting them involved is likely to fail because of that.