Hey Tom,
Thanks for letting us know what are you trying to achieve here.
Currently, we do not have an option to allow admin to add custom privacy from the blog post at this point of time.
The current behaviour is when you had enabled 'Integrate with JomSocial privacy' setting, there are only 4 privacy options for it which are Viewable by registered members only, Viewable by all users only, Viewable by friends only and Viewable by me only
And it is not easy to implement this even if you would like to add custom code into Easyblog core file mainly because JomSocial itself doesn't have this custom privacy.
Imagine if user publish a blog post, so it will generate a stream item into JomSocial wall, so this stream item itself only can handle 4 default privacy, other user still can able to view this stream item even you set only family member can view this blog post.
In other word, if you would like to customise this, you have to add custom code from the JomSocial core file as well.
Thanks for your understanding.