Ultrasonic Frequency to distance
The sensitivity of ultrasonic transducers does vary over frequency. The following example is from a Kobitone 255-400PT16-ROX datasheet. It would also be fair to assume it would fall further outside the normal operating frequencies that are shown below.



However in my opinion using it do determine distance isn't likely to be a good solution. When operating outside normal ranges you may find differences in performance between parts but for experimental / hobbyist purposes I see no harm in trying it as a simple solution.

Another way you could go about it is to change the amplitude of the signal at the ultrasonic emitter end and encode it with a digital signal that indicates the transmission level used. That does get quite a bit more complex though. If you wanted to take a look at going that way perhaps the VirtualWire project would be worth a look at for encoding and decoding the digital signal.