Greetings Peter:
Well, everyone is entitled to their opinion regarding timber quality -- believe me, I've heard a lot of conflicting opinions. However, conflicting opinions often provide valuable talking points.
The best walnut stands I've ever come across were natural stands. Most of these natural stands originated on well-drained, fertile alluvial bottomland sites. The biggest, most spectacular walnuts I've stumbled upon were on mesic, upland or cove sites with loam-textured soils (pH 6.0-6.8) and with high base saturation and cation exchange capacity.
What most timber/veneer buyers look for when buying high-quality black walnut logs: uniform texture (important), centered pith (important), narrow sapwood (sometimes an issue -- sometimes not an issue), uniform color (varies by buyer), no peck or other external/internal defect (important).
I personally believe that you shouldn't have any problems marketing your trees when the time comes to sell "if" you can produce high-quality logs in a plantation-like setting! Moreover, I don't believe you will incur a "penalty" of more than 5-10%, if any penalty at all. Log/tree values always revert back to log quality and demand!
Best of luck!