When not logged in, if login / signup would let me comment, I should see a breadcrumb

(User Story, Investigate , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Conversations, User Interface
Summary: This is one of the major lures to join, so we want to encourage folks to do it.
This story is a side-effect of fixing The "add a comment" fields show up even if commenting is turned off on the Space. At the moment, you don't even see the comments if you're Anonymous and there aren't any on this Thing. That's intentional -- if the Space has comments turned off, we shouldn't show them at all -- but the result is that an Anonymous user (or a non-member of this Space) often can't even see that there exists such a thing as comments.
Fixing this is necessarily subtle. We want to show some sort of "sign in to comment" or "join to comment" breadcrumb if and only if this Thing allows comments at all. So we may need to do an unusually subtle test on the server side, to determine this flag.
It's also unclear what this breadcrumb should be. Should it be the standard comment box, with alternate placeholder text? A button? A link? Needs thought and design.