I used a bit of what I would term an exploit to generate my initial quantity of soul sand. Browsing thru NEI, soul sand seems intended to be made in a Barrel, by combining sand with some kind of Witches brew... which in turn is made in a Barrel by combining water and mushroom stew. Thus, I set out to make mushroom spores via Agricraft for making that stew.
However, I soon also realized there's another way - by putting Railcraft Blast Furnace Brick into a Rock Crusher. Most likely, the Railcraft recipe used to require soul sand (prior to Expert-mode recipe changes), and therefore the Rock Crusher outputs both Soul Sand as well as Nether Brick. But that's an exploit in some ways, because you don't use Soul Sand to make the Blast Furnace Bricks, all you need is TNT (gunpowder and sand) and regular clay bricks. Usually, when crushing (or otherwise recycling) items, you get back what you put in (in terms of its recipe). In this case, you wouldn't get much back at all besides sand, since TNT emerging intact from the Rock Crusher would be pretty LOL.
Sorry if this ruins anyone's fun if it results in a recipe change. I can't say I was disappointed to utilize it!