Because the HDAudio lets you use any jack for any purpose, you'll need to specify which jack you want to be Line In and the registry changes will probably be extensive.
Use RegShot on a clean system, take a snapshot, plug the Line-In device in, set it to the volume you want, take another snapshot, and let RegShot compare the two. You'll get a text file with all of the registry changes which you can save and then push as a machine startup entry. http://sourceforge.net/projects/regshot/