Boone's Battery, Johnson's Bn, and Dubecq's Cav were all made up of a lot of Avoyelles Parish men, so if he was in a CS unit, any of those would be likely candidates. And Johnson's and Dubecq's both folded up early in the war, so a lot of local men were in those units, then in other units later. I'm amazed Seiss would have applied for a Confederate pension. Have you checked to see if he tried to milk both cows?
The CD I got with Joseph Laprairie's records - also a La. Scouts Bn man - was incredible. It's like looking at the original papers, maybe even better. Whoever is doing the photography/scanning on these things is good. I strongly recommend your Military Records Research Service to anyone looking for National Archives records.