Yes, I agree... hickory will work fine. I've done it, and also used maple backing with walnut. 1/8" is perfect for hickory, but I go 3/16" with the maple. Watch out that you don't get too strong of a backing, or make it too narrow, as walnut can fret if it's overpowered (I know this from experience!). I haven't had good luck with bamboo as a backing for walnut. I also tried an ELB trilam with hick/maple/walnut, and it fretted very bad. If the belly wood is ipe, it's a whole 'nother story.