> I understand that it's the software side of SPI that could throw
> someone, but as comms go, it's about the easiest to pick up

An ad hoc bit-bang would do to pass a few bytes. The receiver
can signal its readiness by changing a handshake line from L to H
or something equally basic. It could tri-state that line and receive
data on it. Simple timing loop if they're on the same clock or even
Manchester. Or data + clock


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