Seriously, however tempting a 1 pin keyboard might sound, I'd steer clear from it. I've seen enough examples of 1pin keyboards going bad after a few years in operation. Whether it is contacts getting tarnished, or just aging of components; whatever. For a good professional product, I strongly recommend a proper digital matrix instead of a 1 pin ADC technique.

The technique itself is workable as you will find out after you are through with your trials.