Added some functions to the sketch running on the Master Arduino to make displaying things easier:

https://gitlab.com/arduinoenigma/seven-segment-art-installation/tree/master/Board100x9x13-Master

void SendCommandXY(byte XL, byte YL, byte CMD, byte VAL)
{
  Translate(XL, YL);
  SetCMD(CMD);
  SetVal(VAL);
  TxSetChecksum();
  PrintTxFrame();
}

Ultimately the goal is to manipulate arrays and have a function that scans what changed and send the commands to display that.