For the radio I would recommend getting a Visteon or Directed HD Radio box and using Mitch's cable to run it. You get a good quality receiver with serial control plus the extra added benefit of HD channels.
For the Volume controls there is a lot of options. You can make one like u said, and connect it to serial or through a fusion brain, or you can buy a USB controlled one with pre-made software like the griffin power mate or other USB volume solutions. Run a google search for usb volume control and see what pops up.