A) The difference between a clean hit and a dirty one is a matter of milliseconds. Players who dish out the hits can do very small things at the very last moment to try to hurt an opponent and I don't think any sort of warning system will help you with that. The time needed to react is way too long and you would need to be able to detect 'malice' in someone's intent, which is not exactly easy.
B) Pro sports players are known to hate additional equipment, gizmos, protection extras, etc. even if it means reducing their own safety. Hockey players get hit in the face with sticks and pucks going 100 mph all the time, but still prefer to not wear a visor because they find it "more comfortable". They don't care if they're completely toothless (they actually see this as an honor - many don't get work done until they retire), as long as they can play unobstructed. Same with braces and pads, players will never wear them (and players' associations will side with them) if it means gaining half an inch on the pitch.
Unfortunately, I don't think any amount of technology will help here. We need to change the way people think about contact sports (someone in this thread mentioned its likeness to wresting) and change the rules to protect the players better.