Lots of necros and no solution yet in sight, so I don't feel terrible about adding something here. The feature as envisioned in this thread would have substantially broader applications than some other suggestions or jury-rigs. Most of the other solutions, such as auras/invisible tokens or Bump, are focused on allowing one player to position, if not see, a token they control. A "seen by" list would allow me to approach my current problem, which is to allow all but one player to see a card or token (not its position, but its contents), while leaving the last blind to the contents. I could rig something up with dynamic lighting or advanced fog of war, but that would leave the "blind" player with an ugly black or grayed-out square on the board, which is undesirable.