Is there anything similar to Google AOAP so that a FireOS app that we write can talk to an accessory connected via USB?
Are you talking about AOA, as described here: https://source.android.com/devices/accessories/aoa2
If so, then regrettably no, there's no similar project from Amazon at this time. If it's not a secret, what are you building, so I could forward your use case to the Fire OS team to review? Or if you prefer, please login to your Dev Portal account and send us a private message via the "Contact Us" form. Thanks!