I need to test this myself, but try something? Unbolt the IAC, remove it from the vehicle but leave it plugged into the harness, start it up... I think the plunger should push all the way out. Now use something (NOT your fingertip, intake vacuum is pretty high and could bruise your fingertip badly by sucking the blood through the skin) to block the hole in the center of the IAC valve plunger seat more and more... don't block it completely. I think the IAC plunger should retract if you do this.
Also, isn't the IAC actually a bipolar wound stepper motor? I'm pretty sure those don't have retract/extend signals, just four terminals for two windings, each is driven by an H-bridge type transistor switching network in a certain pattern to move the plunger in and out one tick at a time. I need to check the schematics in my FSM to make sure but I think that's correct.
EDIT: thinking about this more, this has the potential to damage your IAC by running it up against its stops, and could stall the engine. Be careful, feel free to not do this, I'll be doing it myself at some point as an experiment with a spare IAC.