Terry,
I'm sure you must be speaking of Col. Henry Strong in relation to Sharpsburg. If I recall correctly, I found several years ago that more than a few Louisiana soldiers and from other states were interred in Washington Cemetery in Rose Hill Cemetery in Hagerstown, MD. In 1869 by order of then Gov. Bowie of Maryland, he had these and others researched. From this became what is known as the Bowie List which gives a graphic map and listing of the burials in Washington Cemetery. The men are buried in rows that form a semi-circle and at the head of the Louisiana row is Col. Henry Strong of the 6th La. I have used that as my basis for his being buried there. I used it also to identify some other soldiers listed there with only partial names or spelled incorrectly. There is a map of the Confederate section at this website and it is the same map inscribed on the marker in the cemetery:
http://www.whilbr.org/assets/maps/zoomifyimage.asp?imagefolder=RosehillCemetery

The website itself with other information is at:
http://www.whilbr.org/WashingtonConfederateCemetery/index.aspx

The Louisiana Confederates are on the right hand side of the semicircle.

Now, if you were really speaking of Col. Isaac Gordon Seymour, I'm afraid I never found where he was buried. All I have ever seen is that he was buried on the battlefield at Gaines Mill. His papers at the University of Michigan may reveal more but I have never seen anything else. His son left no other record that I am aware of.