I'm not an engineer.. but some random thoughts?
1) Can you use out of band communications to trigger a special mode of the router (assuming you control it)? (e.g., A special listening mode that gives certain devices priority)
2) Rather than use typical QAM type of modulation, can you use a more limited constellation but BOOST the power so you can punch it through the noise? (which would also allow you to make very good use of forward error correction (FEC); again assumes that you are able to program both the sender and the router.
Assuming you can't alter the Network Stack on either device than you have to look at the communications itself
3) Other than that, it would seem you would need to use a programmable antenna/software antenna, Etc. turning your own Antenna into a high directional Antenna with as much gain as possible. Basically find the WIFI router you want (geo locate it, perhaps triangulation with the help of friendly nearby devices) and the push all your signal towards (a dynamic Yagi antenna that auto-magically maintains it's "aim" at the router even as the sender and/or the receiver move about.
4) If you did have some control over the router and you were sure that you could depreciate other traffic then the router would also get the same auto-magical antenna system and the two devices would "focus" on each other.