Martin,
I've done three of the manifold heat dampers, and the method that worked best for me was to separate the intake and exhaust manifolds. Caution: This can be a challenge in itself, as the 3/8" bolts, studs, and nuts may be seized and break off when you try to remove them. Then you have the problem of trying to drill out the broken bolt/stud and re-tap.
Assuming the manifolds separate OK, you can now see the heat damper. You'll notice there's about 1/16" total lateral clearance between the sides of the damper and the walls of the exhaust port. When the damper shaft is free, the damper and shaft can be moved slightly from side to side. With the manifolds separated, you have good access to get penetrating oil (recommend PB Blaster) onto the shaft, both inside and outside the manifold. Using a hammer, tap the shaft laterally in both directions, keeping it wetted with penetrant. Don't hit it so hard as to mushroom the shaft end or bend it. Once you get the shaft to move laterally, you're pretty much home free because this allows the penetrant to soak in along the shaft. After you are able to tap the shaft back and forth laterally, apply a light tapping to the damper counterweight to rotate the shaft toward the open direction. If it rotates slightly, tap it back toward the closed position. Continue tapping the counterweight back and forth. Soon, it will become looser and you'll be able to rotate it through its full arc. Continue rotating it back and forth while feeding it penetrant. Once the rust is disolved by the penetrant, the shaft will rotate freely and you should be able to resume full automatic operation if your bimetallic spring is still in place.
If you're reluctant to attempt to separate the manifolds, you can still use the same method--you just don't have access to get penetrant to the inner part of the shaft, and you can't see where your lateral clearance is in order to know which way to tap the shaft ends. Even so, if you're patient, you can probably get it freed up using the above procedure.
Mark