Thanks Riva.
You're totally right. 
But I'm stuck on the first hurdle. I have all the manufacturer's data for devices I wish to emulate, so I know what I'm likely to receive (baud rate , actual message in hex and/or ascii).
This data is output from the Controller: (a well known brand of proprietary control hardware)
6B 61 20 30 30 20 30 31 0D (HEX) or ka 00 01 (ASCII)
and what it means is, "Switch On Display" (on an LG monitor)
I don't really need to receive any data back from the emulator, just have it react. Like print a line of text to an lcd display or even just light up an led.
Using this sketch I can read the controller's output.
#include <SoftwareSerial.h>
SoftwareSerial mySerial(10, 11); // RX, TX
void setup() {
 // Open serial communications and wait for port to open:
 Serial.begin(9600);
 while (!Serial) {
  ; // wait for serial port to connect. Needed for native USB port only
 }
 Serial.println("OK");
 // set the data rate for the SoftwareSerial port
 mySerial.begin(9600);
 Serial.println("Ready");
}
void loop() { // run over and over
 if (mySerial.available()) {
  Serial.write(mySerial.read());
 }
}
After that I'm kinda stumped because I've tried various other ideas, like the serialEvent example but with no success, yet.
Is this maybe a post for a different forum, now that with this forum's help, I know that the project is indeed feasible.
Thanks again.