Vicki Betts and I spent 8 working days at the National Archives several years ago, and I have made a couple of trips back since then. I have looked for any material relating to Ordnance Activities in the Trans-Mississippi, and although there are some letters and correspondance, no records survived other than some of the records of the Tyler Ordnance Works, which consist of Hill's letter book and monthly account book.
However, the Smith County Historical Society does have a fragmentary record from the Arkadelphia Ordnance works which lists some names and pay records by month. My recollection is that it is from 1862. I think that there is a transcript, but do not recall. I can check on it and see what it contains and post it here, or you can email me at rgilbert@suddenlink.net. I could tie in a few persons who showed as being workmen at Arkadelphia who were subsequently transferred to Tyler in the fall of 1864.
You might also want to check with the Clark County Historical Association. http://www.clarkcountyhistory.org/
I recall that a number of years ago they had an issue of their Clark County Historical Journal that was devoted to the Ordnance Works there.