I'd say it's too risky for the stage... Of course the decision is up to the director, but I'd fake it.
There is too much chance of something going wrong electrically or mechanically, or an actor flubbing the performance,. Maybe, I'd do it if I could build a back-up... Then, you'd only have to shut down the show for a few minutes to switch-out the prop...
88 inputs is a lot, and you'd have to use multiplexing, matrixing, a serial connection, or some other "tricks" if you wanted to sue a single Arduino. (And 88 switches/wires/connections means 88 potential failure points on each end of the cable/connection.
How much time have you got? I do know a lot about electronics, but I've only done one Arduino project so far, I'm not a musician, and my MIDI knowledge is limited... with everything I don't know, I'd want 6 months
to learn, design, build, program, and de-bug the thing.... Hopefully, it would only take a couple of months, but it HAS to be ready & reliable when the show opens (actually before
the show opens).