Would it be possible to have a strong enough signal that you could sit in a car in the lot or on the street?
That way anyone could do it at any assembly or convention and not have to actually attend.
Not really. WiFi signals have pretty steep drop off in range,particularly if anything is blocking them like walls. You can achieve pretty high range through directional antennas, but you need one on the other end as well. I live in an older historic house with plaster walls, I have to use a NightHawk R7000 router rated for 8000 square feet just to cover my 2600 square foot house and get consistent signal throughout.