That wouldn't be crazy, and it's not much different to what ASAN does. But then what happens if/when a subsequent call to malloc() reuses that memory location?

That wouldn't be crazy, and it's not much different to what ASAN does. But then what happens if/when a subsequent call to malloc() reuses that memory location? That location's entry in that global table is given a new maximum offset, and now the old pointer and the new pointer are both pointing to "valid" memory and you have memory corruption as they stomp on each other's data.