Here is a quick sketch of the modified DMX with PIC receiver.
The PIC can be set to one of 32 panels, we only need 28. The PIC is pre-set to expect to receive 25 bytes of data starting from the pre-set panel number and to ignore all other data. The PIC doesn’t store the data, it simply provides the clock to allow the data to be entered into the shift register. The data is fed to all shift registers in every panel, but only the panel with the active PIC will provide a clock pulse to clock the data in. When all 200 bits of data have been entered, the final job of the PIC is to generate a latch pulse to update the display. Then it waits for a reset signal before repeating the process. And that’s all it does.
As you can se it is based on the DMX data format but with all the unnecessary bits removed. If we assume that each receiver always expects 200 pixel data, then with 28 panels, this gives 5600 pixels maximum, of which we will use only 3700. But DMX maximum is 512x8=4096, so this would also be different from DMX standard. But the advantage is that the receiver PIC is as simple code wise as possible.

Had a quick word with bro and he should be able to do this. I will need to construct a simulator so that he has something to test the code out on. This will not be difficult to make, and in fact would come in handy when testing the actual display and panels.
Regards
Rob