Bill,
From the tone and info of your post it sounds like you know a bit about audio production. However, you don’t have all your facts straight in this case. The Canon 5D does have manual audio controls; it has since a firmware update in March 2010. You can disable the AGC and in manual mode you can adjust the levels. If you turn the levels all the way down and use a good preamp you can record good quality audio to the 5d.
And there are many pros who do just that.