Wow. Put this on the GitHub because if this is true, it’s a major flaw and harmful. Members may have to a) be removed until fixed b) fixed immediately.
Are you certain Members data is not structured to support this?
Concerns about cloaking don’t apply because Ghost doesn’t put the whole post contents in the page and hide it via JS/CSS.
Members-only content in Ghost isn’t available publicly anywhere, even to search engines. Logged-out content is served exactly the same for visitors and search engines. The only way to see protected content is to be logged in as a member.