Amaterasu is a projectile weapon. Even as such, it is an incredibly fast moving attack; however, this does not change the fact that it cannot shoot inside of a body just because one can see through it. One cannot go through without dealing with the wall itself.
In addition, the Byakugan is renowned for its insight where as the Sharingan is renowned for its abilities that are given to the holder. At best the Sharingan is capable of seeing chakra. At this point it would have been mentioned if this was true...or Madara, deciding to kill Tsunade first, would have used this on her.