Can ultrasound be used to determine distance without line of sight?
If both devices could both send and receive... then they could talk to figure it out.

One would send a reference pulse, and the other would respond as quickly as it could. As long as your timing on processing and sending was fairly predictable (low jitter), the first device could take the total time from sending the reference to receiving the response, subtract a constant processing delay that would be very easy to experimentally measure, and arrive at the trip time.

This would not give a direct distance, but the distance going around any obstacles... unless the obstacles were particularly transparent to ultrasound. Even then, the difference in propogation speed through different mediums would mess with your results. Practically, it would most likely nearly always result in the through-air trip time around obstacles.