Are you going to be singing for the performance? I mean-- are you going to set up the gear and then run it by remote control, as it were?
I'd say your instincts are excellent, ORTF over the conductor's head and a ribbon right on the piano is a tried/true approach that invariably works well (on the theory that what the conductor hears is hopefully not too radically different than what the music is supposed to sound like) and (ribbons are great just outside the crook of the open/semi open piano lid).
Fine tuning the gain is one of the little nuances that serves to optimize the whole ordeal, though... be sure you get a long and drawn out sound check, to give yourself an opportunity to do this, jiggle the mic placement, do all that you can to optimize the gear. Good luck!