I'm still waiting for someone to make an app that does this -- would it require linking 2 phones?
Additionally for their experiments they needed at least five antenna, so even if it were going to happen with phones you'd probably need more than just two.
Xbee hardware can be co-opted into Wifi transmission by an Arduino or more powerful processor: https://github.com/cjbearman/xbee-wifi-spi-arduino
I wonder if Xbee devices would be more suitable to this task.
It does not cause 2.4Ghz 802.15.4 XBee modules to act like a normal wifi (802.11) chipset.
Just an FYI.
If you look at one of the early WLAN papers  (I'm an author), Figure 1(b) shows a frequency selective fade, meaning there is a dip, or null, in the received signal and some of the frequencies are missing. The way we created that null was by a person walking through the room. With a change of emphasis in the signal processing, a WLAN is a very good person detector.
knowing how many people on the other side of the wall will change our sense of privacy
It is possible that this tech is especially effective at tracking humans due to the fact that we're bags of mostly water and the specific frequency used interacts with water more than other nearby frequencies. (I'm not a microwave engineer, so I don't know this for sure)