If you don't want to spend much on sensors, interface, etc, instead of sending MIDI maybe you can use a contact mic on the floor. It will pick up the footsteps of people, or you can use three and triangulate the position of footsteps.
A year ago I built an soundinstallation called the KosmiX
( http://www.dannydegraan.nl/ and click on installations ) with some friends of mine with 64 light sensors. They were mounted in the floor so that kids could stap on them so they could play sounds. We used MAXMSP for the processing and 4 eobody's from eowave http://www.eowave.com/ for the sensoring. They have a real simple sensor-to-midi translator with a whole bunch of different sensors ready to be plugged in. They are relativaly cheap and you don't need to solder anything.