To be honest, I had heard there were more than a few unknowns at Camp Douglas. That place was worse than Andersonville having operated with the blessings of Abraham Lincoln himself. I have also heard that many of gravesites were lost with the development of Chicago.
In my opinion, it would appear the country down around Arkansas Post would have been so rough that if the opportunity came up to run, I feel sure the Confederates ran and could have escaped because of the lay of the land. As your person was taken in Arkansas, it is very possible that the details are in a book called Rebel's Hell as most, if not all, of the prisoners were shipped out of Little Rock, so they ended up in the Yankee Prison there.
The book Rebel's Hell was written from records at the National Archives.
My first cousin's ancestor fought at Arkansas Post being a Union soldier from Illinois. The man's brother was killed there. The man himself later lived there marrying and taking a wife from the locals and raising a houseful of children. Can you imagine the surprise of his descendants to find out that he was a Yankee? The real kicker on the deal is that her husband also descends from a man who served with the very same unit.