Sure you could use a matrix switch, but if you're going to all that trouble you might as well use an Arduino to do it with. Another option is to use multiple Arduinos running the MaKey MaKey sketch (with added 22Mohm resistors), or multiple MaKey MaKeys (which of course run a version of the Arduino bootloader), plugged in via USB at the same time. Then there's no need for a matrix. They can all act as seperate keyboards of 18 keys each. I have tested this with 3 MaKey MaKeys, though I haven't tested this with multiple Arduinos. But either way it should work. While in some ways this could be simpler, it costs more!!!