I'm not sure your assumption is correct. I think the bookmark
is just a little piece of "find me here" code in the topic text.
The only tight connection is that the topic filename is part of the
path to the bookmark.
The Project pane does display bookmarks as if they were
sub-topics, but the Project pane displays other things that aren't
properties (like image map links). So I don't think that's a
Bottom line - you're a pro (I recognize your handle), and if
you can't find a setting for this, there probably isn't one.
I wonder if one of the reports might help you manage your
bookmark index links?