Thanks to all for the replies.
Once I got a MAX3232 chip/board and a (SparkFun) Level Converter, plus two 10K ohm resistors to give me 1.8V for the level converter from the 3.6V supply, I was able to hook up my embedded Linux board to the SL8092 and get it back to USB mode using ‘AT!MAPUART=7’.
I now have a useful serial comms wiring diagram! 