There is no concern weather you are using V2 or V1 calls to access the messages from Private category. Its just a matter of how you are processing your request by using it in FreeMarker code via the rest and restadmin context objects.
More documentation here.
To Solution to your concern follow these steps:
1. I recommend you to create a new role on your private category or board which ever is private.
2. After that give new role permissions you want like view,read access to board, discussion.
3. Assign that role to the users whom you want to show the messages from private area.
4. In code, ask your dev team to process V2 Rest API requests using FreeMarker code via the rest context objects and don't use restadmin.
