You may be surprised to find that you can get microprocessors with over a hundred general purpose I/O pins if you're willing to solder them all down (the STM32Fx series in the QFP208 comes to mind) that would let you wire every key of a 104 or 107 key keyboard to a pin for monitoring. And you could read them all at close to a megahertz so you could get an effective state of all the keys at a given point in time. (the chip can run at 168Mhz and you're looking at basically 14 I/O ports.)