Here is where you have a problem. "but if its border line"..........so who decides if its borderline?....the umpires of course.
If its borderline, then it becomes judgement. Any each umpire has his own judgement. It wont matter at that point if the last 2 umpires didnt think it was illegal when the 2 umpires today judge that it is.
I would not want to be doing something so radical that only 2-3 pitchers in the country are trying to get accepted as legal when it counts.
I may not be imaging this correctly. As I say, I'd need to see it. But it sounds like a duck.