I successfully managed to use the ports CE (A0) and CSN (A1) as a 0-5V voltage meter. But there is a strange 1.2V reading on both ports without any cables connected and a strong effect where a voltage increase on one pin increases the readings on the other port too.
Everything works fine once I connect the CE & CSN pins to either GND or VCC, but it still leaves me puzzled about what is going on.