We deal with this a lot in our band, though we rarely have an upright piano available to us on stage and use a keyboard. I guess the first thing you could try is rearranging the ensemble so the piano player is in a position to see you better, even if it's by looking to one side. Could you stand while you play so you're visible over the top of the piano? Yelling works in a pinch. What does the piano player say would help in this situation?