Harvesting power is a bit questionable, but in terms of an input mechanism, Square built their reader on the idea.

As far as it being a hack for a closed platform, I'm not sure why that matters if your goal is to interface with the primary mobile phone platforms. This solution works, and if you need something more professional, working with the dock connector itself isn't an burden any more insurmountable than dealing with injection molding or other product design and manufacturing concerns.