I've wondered about this, too, and wonder how they got away with it. Gotta be shallow, and frankly, paints or dyes seem easier.
I considered chasing down one ring and leaving the winter wood on the surface. Just a thin layer to work the design into, but with a strong summer ring holding the real tension load. Or leaving that last layer of inner bark? These days I could glue it down or consolidate it with glue or wood hardener, but how about back then?
I will say that if the tiller is PERFECT and the stave perfect and clean, that would help.
Also, I took the bark off some plum last fall, and the whole first ring was eaten up with the squiggles of some small borers, nearly 1/16" deep. " Ruined for sure" I thought, and tried to break it over my knee in disgust. It chose not to break. It flexed, took no set, really, and didn't pop anything up or flake off the front like I would have expected. WTHeck? So, I floor-tillered it, but haven't messed with it yet.