has anyone used straw bales as retaining walls before? This may be a
complete no no but I'm interested in how bales would perform sat
directly on and up against soil with the external faces rendered in
a breathable plaster.
I don't think that's going to work well. Forget about the breathable plaster--what is your plan for the inside of the wall, the part that is in direct contact with the soil? Water happens, even in the desert. (I don't know where you are because you haven't put that into your profile. You don't have to give your address, but it helps to know where you're coming from.)
try using a gabion, its a mesh cage filled with stone. Or you could try using plant roots.
How steep is the "cliff" 90 degree and how high is it. how much rainfall do you normally get.
You could also try sand bag, aka textile cage filled with sand.