Seems like a sort of OOK might work...
a. IF the device can be hacked to make a transmitter that can send the normal 8 pulses at any time (and not wait for an echo), and
b. IF it can be hacked to make a receiver (by the method mentioned in the 3rd para above in post #4).
After some initialization/synchronization transmission, to tell the receiver to get ready, then communication begins at regular intervals, where a train of pulses = 1 and silence = 0.
If slow OOK would ok, the hack mentioned in a. might not be necessary: would have to wait for the transmitter to time-out (from not receiving an echo...about 38 ms) before transmitting again. That would control the interval timing.