Hi Geert

I assume that there will be more than one transmitter (TXS).
If they are not synchronize and if two of them transmit at the same time at the same frequency (i.e. 56Khz ) you will be en serious problem.

Perhaps you could implement same kind of polling, but for this you need that the TXS could also receive RC5 code.
Or just make a simple circuit in each TXS to detect if "the air is clean" to start the transmition.
Just an idea.

Good Luck
Daniel