The Vibrant ultra quiet that I linked above is "designed" to reduce rasp.
The pattern jewc is referring to are called "louvered" resonators. They are typically found on cherrybombs and other glass packs (for example, summitracing and jeg's have a house-brand series of glass packs with louvers).
The louvers are meant to reduce noise in general, not necessarily rasp. It's a different way of reducing noise, it creates turbulence in the pipe so that the sound waves will bounce and cancel each other out. Other resonators use perforated cores, which allow the sound waves to travel from the exhaust stream into the canister, where they are absorbed (usually by fiber glass, steel mesh, or some other material). Louvered mufflers reduce flow by about 2-5% depending on the size, when compared to their non-louvered counterparts).
To reduce rasp, you're best off with a chambered muffler to begin with, and if that's not possible, then I would go for a large body perforated core, just like the Vibrant ultra quiets. The "bottle" style resonators, which are much thinner, don't work as well for rasp.