Other than lookup, there'd be two ways to do this: via a translator that runs on every page, as we do for DOIs, and via right-click. We could do, either, really—or both—but we'd have to verify the check digits, since ISBNs otherwise can't be reliably identified the way DOIs can be.

We could require "ISBN" (and optionally some punctuation) before it. It seems like with ISBNs that's going to be there the vast majority of the time (with table-based layouts being one exception).

Doing it via the context menu—showing the option only when the check digit passed—would be pretty easy. We have an old ticket for it.

Thanks for showing interest in my idea, I look forward to seeing it realised :)

If I may, the ticket for this enhancement looks like it was opened four years ago and last modified two years ago, but not yet resolved. If I'm missing something, my apologies. If not, seems like a long time to wait for an improvement that should be relatively simple to implement, yet with results that are extremely powerful to users.

It would allow users to circumvent translators they don't like, or operate effectively where none exist. The user knows when she sees an ISBN -- she highlights it, right-clicks, and voila, import happens (in all likelihood) with reliable data via an LoC lookup. What could be better than that? No cut-and-paste, switching windows, etc.

Short story: Getting this into an update of Z soon seems like a lot of bang for your buck.

While we're on the subject, I'm wondering why Z doesn't allow for batch-processing of identifiers (an import option akin to the magic wand that accepts multiple identifiers rather than one at a time). That too would eliminate many, many keystrokes and mouse clicks for a user who is importing, say, a lot of ISBNs at once. Often, those are new users rebuilding their preexisting libraries from other platforms who, presumably, you would be enticing all the more to adopt Zotero.

I don't think most people view this as super high priority since you can do this very quickly by copy&pasting the ISBN into the add identifier window, but as ajlyon and Dan say above, help/patches are welcome. There are a lot of open tickets that are many years old - it's faster to come up with ideas for functionality than to write the code ;-).

Batch entry, which I think is a much bigger potential time-saver, is already coded, the patch for it is here:
https://github.com/zotero/zotero/pull/164
and I'd expect it to make it into the next major Zotero release (3.1/3.5).