Especially as the 8051 serial port has a 9-bit mode specifically designed for multiprocessor comms!
And variants with the Enhanced Serial Port have automatic hardware detection for the start of a frame (when b9 is set).

In fact, as I said originally, I can't see that this protocol requires any specific hardware implementation - it could equally be done on a serial link. I think the "async parallel" is irrelevant to the protocol (I'm not even sure that the suggested hardware implementation actually is asynchronous).

In protocol-speak, the hardware implementation is Layer-1, and the protocol is Layer-2, and the two can be treated independently.