I built a tiny 9 7/8" scale length uke with an all walnut body. It sounds great. It was straight grained quartersawn wood. I wouldn't hesitate to do all-walnut on a bigger instrument based on that experience. I have used walnut a lot as a back/side wood, but that was the only time as a soundboard.
How stable is the crotch grain walnut you have? I've cut plenty of crotch walnut for furniture and fancy laminate tops on solid body guitars and basses, and it has been among the least stable wood I've ever worked with. It seems to have a mind of it's own once cut, even if it was well dried and carefully handled. I've had pieces twist up, or develop checks years later.