The master needs to know that the slave acknowledges the signal. So we are talking bi-directional signals. Christian said he wants receivers only. How do you square that !
First "superchecksumming" i.e. checksumming every bit, every byte and every record (use CRC where possible). This will avoid a "lamp off" being read as "lamp on" in more than one case of a few million (if needed, someine else can do the math).
Second, in a round robin fashion, send all lamp messages all the time. Transmission problems may lead to a lamp not switching till, say, 10 cycles later but I doubt that an occasional delay of a few seconds in the status of a lamp will kill the project.

Erik

BTW Christian, if the lamps are tungsten, do remember that the resistance is very low when cold (your switch need to be able to handle the inrush current)