Converting 8 bit binary to a hexadecimal pair as unsigned char
As StarCat suggested, I can just send decimal values using the same function SendData I was provided with. Because the e-ink board can only communicate in 8-bit commands I still had to split my decimal value before sending it.
This is the function I used to send decimal data in 8-bit pairs:
void Epd::SendDecAsData(int dec) {
  if (dec == 0) {
    SendData(0);
    SendData(0);
  } else {
    SendData((dec-1) / 256);
    SendData((dec-1) % 256);
  }
}