Two ways come to mind and you may want to experiment to see which will work best for you. The first is using masking tape and gel stain. That should be fairly straight-forward as long as you are careful wiping the stain around.
The second method is to dye stain the piece with the lighter stain color using water soluble aniline dye stain. This material is available at specialty woodworking stores or via mail order on the web. When you have the piece all one color, then do the masking tape and oil based stain process.
I would think that the second method will be better with a wider color variation than you could get by with using the first method. At the same time, the base color of the walnut is not going to allow a real wide color variation anyway.
For what it's worth, I believe that it was the last issue of Popular Woodworking magazine that had an article by Bob Flexnor on finishing walnut. You might want to check this out.
I guess that you could also try tone sprays but I think that the amount of color shift and the probablilty of some of the spray getting under about any releasable tape would be pretty strong. If you don't mind some variation in the line, you could make up a cardboard mask and hold that just above the wood while you shot the tone spray straight on. This would work better if you had a reliable helper for one job or the other.
Please, let us know what you tried and how it worked, would you? I doubt that you will get anything that really approaches "tiger walnut", if that's what you are going for. I think that the "tiger stripe" really has to be in the wood first to be effective.