[kovidgoyal]

Here is the code for a python template:

python:
def evaluate(book, context):
        from calibre.gui2.ui import get_gui
        v = get_gui().library_view
        row = v.currentIndex().row()
        if row < 0:
            return 'no'
        try:
            return 'yes' if v.model().db.data.index_to_id(row) == book.id else 'no'
        except Exception:
            return 'no'

Then in the emblem rule simply check if the column base don this template is yes. Note I havent actually tested this, but I see no reason why it wont work.