Well, a "signer app" can have a signer and a client. Those don't have to be one thing, it'd just be nice to be able to click on the browser extension and start typing.

Could also be a separate browser extension app, I suppose. I just thought of signers because everyone already has one and mine contains my nsec and my relay list.

Alexandria has this, now, on the event page (search for an event id, get the result, click "comment" type, submit, done). But that takes up a whole webpage, so you need to tab back and forth. An extension is more handy.