[Xymist]

I don't think that scenario is particularly tricky. If you modify someone else's pacemaker, it's a tricky question, even with their consent. If you modify your own, absolutely nothing should stand in your way beyond a nice big notice saying "danger of death,on your head be it". That is, you should have the same freedom to screw with your own personal medical devices that you have to climb out of your own fourth floor window.

People have a right, albeit not enshrined in law, to do stupid things that might kill them - at least as long as they don't then ask someone else to save them.