I was able to move the prop levers into feather position with a joystick button by mapping the button to send the value -3260 to the SimConnect event PROP_PITCH1_SET. I use the Axis and Ohs program and added a script for each prop like this:
-3260 (>K:PROP_PITCH1_SET)
-3260 (>K:PROP_PITCH2_SET)