When creating a new message board post there are only 3 options to select from for view permissions.
However, there should be custom roles to choose from as well.
1. Create custom site roles.
2. Add message board portlet to page.
3. Create new thread post.
4. You can only select, Guest, Site Member, or Owner from view permission drop down.
5. After publishing new post, click Actions > Permissions.
6. Here you can choose which custom roles also get view permissions.
7. Requesting these roles all be options in the in the initial creation of the post.