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My idea is similar as here:
http://www.jrok.com/project/speckey/spec_key.html
You can rid off one wire by using I2C. Since it will be interface with only one direction, there will be no problem with speed. If you want to transfer 72 keys 100 times every second, you will require only 72*100=7200 bits/s. It's payload. Data link speed can be 9600 bits/s and you can insert some extra bits, for example, reset button state.