The thing is, though, it couldn't have lasted longer than it did. By that time, Sherman had long since reached the Atlantic and split the rebel states in two. Lee was only one or two weeks away from surrendering. The CSA was in its death throes, and General Order 14 was made entirely out of desperation, too late for it to have any effect.
I agree that the black recruitment law was passed too late to have any effect. That is obvious from OTL.
Had the situation not been so desperate (ie, an earlier POD results in a stronger rebel position), there's no way General Order 14 would've passed earlier on. Consider the fact that Cleburne supported African Americans in the rebel army, but he was told, flat out, that it wasn't going to happen.
It is true that the black recruitment law of March 13, 1865 was passed out of "desperation" (or, to put it another way, military necessity). Of course, we must remember that the Emancipation Proclamation and the laws which allowed the Union to recruit black soldiers were issued under similar pressures. The North was perceived internationally, at the time, as losing the war, and Britain was considering recognizing the South as a result. So basically both sides reached out to the black population when they felt it militarily necessary to do so...the South just took longer to arrive at that decision.
General Order 14, however, was a War Department regulation to govern the recruitment and use of black troops, not the actual legislation authorizing their use. General Order 14 actually conformed to long-standing Confederate policy with regard to the use of new recruits to rebuild existing units, rather than forming new regiments with them.
Is there text of General Order 14 floating around? I looked it up (never heard of it before), but could only find references to it.
I don't know if it is online anywhere. It is reprinted in full in Lonnie Durden, THE GRAY AND THE BLACK: THE CONFEDERATE DEBATE ON EMANCIPATION, Louisiana State University Press: 1972. This is an excellent book consisting mostly of reprints of original documents...newspaper editorials, legislation, War Department General Orders, political speeches, etc., relating to this issue.