I absolutely agree with a lo-fi, low volume playback. I've done it in a pinch with no warning from production. We had a pianist play a "circled take" which I recorded properly, and we simultaneously recorded it with an iPhone in Voice Memo mode. Really. For playback, we set the phone on the piano, out of frame, low volume. The actress mimed the piano, and I recorded her vocals. Worked like a charm! When it aired I was quite happy, well, nearly shocked, with the result.
Since your application is live, I might suggest that the actor and your pianist maintain a line of sight to each other. They can read each other's subtle cues this way, you know, like actual musicians! If the set doesn't allow a physical line of sight, you could involve the camera dept to provide reference feeds. I am imagining your actor being able to see two monitors out of frame with a medium shot of your pianist. Almost as if they were prompter feeds. It could be a fun day!