Ive never done it, but i bet its the same as final four and spurs playoffs, where most pickups are from traffic jammed market and commerce. I remember during final four, the last go of the old surge, that you could sit at houston&281/37 and put la canterra in destination mode and get a 50$ ride out of there...but that only worked twice.
My best day in san antonio was during the BBQ cookoff running XL only. It was busy but not traffic jammed because everyone was NOT downtown or the ATT center. $10-12$ xls back and forth all night.
If you could find a scenario like that for fiesta and find a way to never go downtown you could make money. But your first trip to market street and you can kiss an hour goodbye. If you're gonna try....at least spend a day beforehand and map out some routes in the neighborhood streets that run parallel to Broadway. Good luck.