Roy Underhill (of New Yankee Workshop fame) fairly regularly uses saw steel for homemade scrapers--not generally card scrapers, but specialty molding scrapers that he hand-cuts. The point of using saw steel, as he explains it, is that it's the right hardness, so I take it it'd work just as well for a hand scraper. But one other variable would be thickness. Some old saws have pretty thick steel, which would be hard to bend (a hand scraper or cabinet scraper both work by flexing, whereas molding scrapers, from what I can tell, stay straight and cut more or less in the same fashion as a lathe scraper--stiff, no flex).
Once you start using card scrapers, you'll see a lot less sandpaper--they're a real treat to use. I highly recommend getting one of the french curve scrapers, too--terrific for coves. (Woodcraft, I think, sells a set of three: rectangle, french curve, and curved end. I think I've seen this combination elsewhere, too.)
