Limescale is a problem in areas with hard water. No more so than in kettles which need occasional de-scaling.
How about making a dispenser which sits in the kettle and provides de-scaling acid in very small quantities. The acid would be food-grade. It would be adjustable to give just the right amount for the hardness of water so that the acid is completely used up in dissolving the limescale. A testing kit could be sold, or a national database could be built up giving local values based on post-code. This would tell you what setting to use on the dispenser.
The dispenser would just sit in your kettle and do its job until it was time to change it. It could either be some kind of burette that clips to the side or a widget-type ball that floats in the water.