Any ideas for a swing-lens mechanism?

Any geared motor should be capable of doing what you want. Common examples of what I am speaking of are the motors that drive appllance (washers, dryers, dish washers) timers. Beyond that Huebsch commercial clothes dryers have a couple of different gear motors. One is a 3/50 ratio (fifteen minute 360 degree rotation) and another is 1/60 (sixty minute 360 degree rotation).

Suppliers of these parts are Johnstone Supply among others.

Once the speed of rotation is known then the aperture would be adusted based upon existing light conditions.

The problem is that these are driven by 115 volt 60 hz power. However these are small amperage devices and a common 12VDC to 115 AC power inverter would handle the load with no problem. Walmart sells the inverters.

I look forward to seeing the results of your efforts.

 

. I imagine the hard part is keeping the swing at a steady rate. Anyone have experience with this kind of thing?[/QUOTE]

My experience is looking at home made things.
The spring typically is called [I think] a "constant force" spring.
It is a very long flat stripwrapped into a coil, so it has the outline of a thick disk . The application uses only a small part of the total , er, winding. That means the force at release is very close to the force at the end of the swing, so speed is pretty constant.
A short spring, like a rubber band, would provide full force at the start and zero at the end, and obvious exposure variation. If one leaves some room between the initial position and the opening of exposure, the acceleration from zero to full speed will not significantly affect exposure. You can find the type of spring to which I am referring in industrial catalogs, etc. McMAsters-Carr probly has them, or google spring.

 

This should help: http://www.cyberbeach.net/~dbardell/panoptic.html