How can I override SpaceView3D.lens 250 mm limit?
Why do you suppose the developers have imposed that limit?
I read http://wiki.blender.org/index.php/Dev:2.5/Py/Scripts/Cookbook/Code_snippets/Interface
but It seems like I should be able to poke the value in through the python console rather than replacing the view panel.
In order to match a mesh in the 3D perspective view port to a photograph taken with a 400 mm lens I change the opacity of the blender window and line it up with the photo in another window behind it. I know I can accomplish this with the camera view and easily change to the 400mm and beyond but I find it a little cumbersome.