There's any implementation possible which does not violate with existing features of the RDS encoder. The PIRA32/P132 uses two radiotexts, RT1 and RT2. Currently only RT1 is filled by UECP. It is possible to fill the other RT in the case that previous RT is already filled and buffer configuration (bits 6..5) is 1 0. This will effectively maintain full compatibility with original UECP RT command if the number of Radiotexts in the bufer does not exceed two.
It would be nice if you can provide me a sample of UECP data for further analysis so I could issue a binding opinion. I can provide a utility for saving serial data.
Let me allow a short comment: Original special arrangements defined in the UECP, like "spinning wheel" or "RT buffer" are deprecated. They have never been used widely, especially after internet connectivity has became available everywhere. Today's RDS encoders are not limited to be only simple UECP machines, there's an independent set of ASCII commands, special text features, volatile and non-volatile memory etc. Keeping the RDS encoders based on old UECP arrangement is not eligible. For anyone writing control application it is convenient to send data transparently rather than filling and flushing buffers and rely on correct transmission.