Good idea! Until I reach the point of having everything hooked, identifying hooked items easily will be very useful so I don’t keep invoking Hook to see if I’ve already created a link.
In my bibliographic software, Bookends, it’s important for me to know if I have the ebook or closely related material in Devonthink (my own writing, usually) so I hook to it (if it’s in Calibre; I so wish it was possible to hook to Kindle) and then use keyboard shortcuts to add a comment at the top of the notes. I have wondered about dedicating a user-defined field to this. I’ve also started linking to entries for books I own, whether physical or ebook, in LibraryThing, so I can know (a) where I need to look for it, or (b) it’s a waste of time looking for it as I don’t have it. I couldn’t use the emoji idea there, of course, as it might be a bit of a problem if they appear in my bibliographies!