I don't think you will want to use engineering or architectural units when in Civil 3D. It is simply not made to handle them. You can force the drawing units to be inches, but I think a lot of settings and calculations behind the scenes assume feet or meters. You have probably already seen in the Units and Zone tab that the only options are Feet or Meters....if you force this to Inches (with "-dwgunits") and then pick the zone you need....you'll notice the "Imperial to Metric Conversion" setting is still using Feet and Meters. I think you would have to somehow find a way to include "US Survey Inches" which to the best of my knowledge is not available. Bottom line, you can trick some things in the drawing, but it will inevitable come back to haunt you....or whoever recieves the file after you. Here is another link of older information, but I believe it is still valid for all versions.
p.s. as I do structural drawings, I too would love to use foot-inch input, but I gave up and left it to phony architectural dimensions with 12 scale and decimal dimensions with a ' suffix.