I think redwood tends to be a "wood of opportunity" for some builders on the West Coast. In extreme northwestern California there are a couple of stump millers who go out to private landholders and bid on stump wood. Most big redwoods were felled with 3-8' of stump left in the ground, which then resprouts with wild abandon. Stump wood is a niche market; most people who want redwood want longer lumber. But if you just need a 20" long board, old-growth wood is still available without downing any new trees. You just have to know where to look -- easy to say if you live in the area. I bought some stump wood from a miller in Crescent City, CA for a small greenhouse some years back. At the same time, I searched through as many of his wood piles as I could, looking for prospective soundboards. I found nothing but wide growth rings and very soft wood, frequently with small pockets of punky peck that aren't always visible upon casual inspection. Some great guitars have been made with redwood soundboards, but as with so many "alternative" woods, it is the exceptional flitch, not the typical one, that will have the characteristics sought after by builders. Most lumber yards don't mill their wood in a way that yields boards that will be acceptable for resawing into soundboards. There will be just ONE board from each flat-sawn stack that has any potential at all.
Not only would a typical piece of quarter-sawn redwood not make a great soundboard, it would most likely make a substandard one, IMO. But if you manage to find that elusive needle in the haystack, by all means, go for it.