I think you'd still have to hack the routers to provide the external clock, as the whole scheme depends on accurate sensor fusion between multiple antennas.
Not only you have to hack the routers, once inside the router you'll have to flash a custom wireless firmware, or "radio" firmware. And also the Chipset must be flexible enough to allow all the iFFT shenanigans.
I think it's a stretch to say you could do this with a common wifi device, maybe with a very specific wifi chipset you could, and maybe you could make it work with bluetooth or wireless phone chipsets aswell.
That said, it's an excellent paper about a soft-based radar.
IIRC, I think there are some weird FCC rules about external synchronization of radio transmitters. Using the clock would be ok, but synchronizing using out-of-band signals is a problem for some reason. (This is hazy, but I remember working around issues synchronizing UHF (915MHz) RFID readers).
That said, it's an excellent paper about a soft-based radar.