The Water Club on the East Side and whatever they are calling that restaurant in Central Park these days, have a light that they turn on away from the restaurant proper where it is more visible to the road signaling that a patron needs a taxi.

Less usefully, but along the same lines you still see doorman blowing whistles to try and get taxis to come down a side street.

I'm not sure how this translates into a solution for your problem, but I think you need to consider some hybrid meatspace / internet mechanism.