Hello. Building an ultrasonic module on the GPIO shouldn’t be very hard. Implement this seems like a must to me. Congrats for the project!
Could be easily doable with a piezo transducer off of a GPIO pin. I would be immensely enthusiastic to try this with the Flipper.
I found this project on arduino uno. Seems legit. https://blog.vinu.co.in/2018/03/ultrasonic-audio-transmitter-and.html?m=1