Is there any way to specify the extent that the Navigation control uses for full extent either by using the ConstrainExtentBehavior or some other method?
I guess you would still need to attach the ConstrainExtent behavior to the Map. The target for the Navigation action 'Full Extent' is the Map and if Map has the constrainExtent behavior then it will be honored whenever full extent is clicked. Hope that helps.
Since you don't want the map to actually zoom to full extent, you should modify the template and remove this button. Then replace it with a different button, that when clicked you do a zoom to your custom extent (you could for instance use the ZoomToAction on that button)
