2 different sized "ball cutters" in a Dremel Tool will work very well. Do a pattern of larger dots first, staying within the lines of the outline you choose, and then fill in-between those with dimples from the smaller ball. Try not to round off any of the "edges" that are created from the dimples and you get a very nice and grip-able surface. You can use stain to match or contrast the cut wood when done.
I added some "Deer Tracks" to a slippery inline .50 caliber rifle using that technique, and it came out great. Here's a picture:
And here a link to the posting of how I did that on that inline muzzleloading rifle:
Custom Grip DIY
Good luck getting a grip.
Old No7