I think we are out-of-luck for a simple solution with Calibre. That app does not support AppleScript.
That said, something might be finagled with Macro-scripting tools like Keyboard Maestro, or by using AppleScript together with Calibre's CLI. Calibre offers a command line interface (CLI) that should allow access to the metadata of each record in its database.
https://manual.calibre-ebook.com/genera ... index.html
In any case, you will likely need to have a ISBN or DOI in Calibre's "identifier" field, which you typically must insert manually. With this info, Bookend's Quick-add function could be fed and triggered. This would be easier than pulling
all the book metadata out of Calibre to fill a new or existing BE record.
I don't have the time right now to write a script or macro like that; I am not sure if it's even worth it, because if you have to manually pull out an ISBN from your Calibre book record in the first place, might as well just use it in Bookends at that moment for a quick-add.
But in general terms, I would be interested, too, to somehow make Bookends cooperate with Calibre.