To the right of the duct entry[as you are standing in front of the car looking at it, left side in standard car parlance] you will see the small rod coming forward and attaching to a small plastic lever attached to the heater door[which you can see inside the duct opening as a steel bar passing across the bottom] I used two small screwdrivers to pry the plastic lever away from the flat end of the metal rod, and as soon as there was a little space I used them to work between the metal end and the clip , since I was nervous of breaking the plastic arm. Eventually the clip broke, they should be a hardware store part.
A bit of gorilla tape should hold it open for the time being. the rod seems to harmlessly move around when detached, may need something to stop it rattling when actually driving
So the tape let go after a few months, and in the fall I realized I could not get the heater off of defrost, so upon examination the lever wedged itself under the heater flap actuator and the servo is not strong enough to pull it out.
So, don't do that.
I spent a stupid amount of time sure I could come up with an elegant solution that both keep the heater flap open and allow the rest of the system to do its thing. Made several complicated looking brackets, then realized I was overthinking it.
Pictured is a small sheet metal strap with two holes in it. A #6 SHC screw pushed through it, then a nut to retain it while assembling. The screw goes through a small hole drilled in the end of the heater flap actuator[with a drill in a dremel] The screw hits the front of the diverter assembly preventing the actuator from closing. Once pushed into place it is prevented from falling out by the end of the pedal box. The arm coming forward from the face/defroster diverter is screwed loosely to the other end of the strap with a hex head M4 screw and an elastic stop nut. The arm now moves as it did but does nothing, just rotates the strap back and forth. It can no longer become wedged. The total travel of the arm is a bit over an inch, so the strap needs to have the holes far enough apart that it can travel that distance without binding.
This is easily reversible by grabbing the SHC screw with extended needle nose pliers and pulling it out, removing the strap and placing the arm back on the peg and putting a pushnut back on it. That would be the hardest part on the Exige, as there is little access.
There is another style diverter assembly with round rods which would require a different mod, but since the arms on that one are easier to snap on and off, one could remove the arm completely, and then restrain the heater flap actuator, or add a longer arm, with no bend on one end with piece of spring on it like Mcmaster Carr p/n 9663k53 so the spring always keep the heater open
BTW i barely missed the heater flap being closed in the coldish weather. I think tightening up the front of the car helps there, there is no cold air infiltrating so the face vents do better work and your feet still do not get cold.