What about the furniture maker making some wooden brackets to match the shelf, and have the brackets painted/stained the same color? It would end up being one piece with the brackets nailed into the shelf?

I have one that we used a long time ago and it really looked nice. That way "ugly" brackets won't be seen, instead it will look like one piece. Which it was. lol

That could be a possibility. Originally he had drawn up a design using two shelves connect by wood "slats" or braces which was too pricy for us, and not quite what we had envisioned anyway, so it was changed to a single shelf. With that change he had proposed having a laser cutter make brackets, but they can't do it now (maybe later?)--also might not be cost-effective for them to make just the two brackets for us....

I'd have the furniture maker build the brakets to fit the style of the buffet so it will feel as though the shelf were part of the plan. Which it is.
I recently finished building floating shelves for a customer from hollow core doors. It's recommended that even with the light weight of hollow core doors they only be nine inches deep. The ones I built were 16.5, but they were set into an alcove with inside cleats on three sides to hold them.
If you really want the appearence of a floating shelf, the best bet would be to ask your furniture maker if he feels there are large enough "L" brakets to hold that size of walnut up. You can always paint the part that attaches to the wall with the wall color and paint the part that the shelf sits on in a color to match the stain as close as possible. Once they're painted and all decorated you'll barely "see" them. Unless you are looking for them.
I had a photo of a friends that added shelves in their dining room like this. If I find it I'll post it later.

I recently added a Pottery Barn floating shelf in my office sewing room. It was for a small flat screen TV and (yes, I'm dated) VCR. I don't replace anything til it doesn't work any more!

The hardware to hang it was purchased at Rockler, as this was an eBay find, and didn't come with the piece that attaches to the wall. I do NOT feel safe with having both components on it, so have now bought two vintage wooden brackets and will make a simple wood shelf to go on them.

I don't know what you plan to put on your shelf, and it may be light enough to go floating. I haven't seen your dining room or the proposed shelf, but agree with the above that it needs to be part of an overall cohesive look. I think of floating as a more contemporary look and have realized it's not 'me'.

I would say that for shelves that deep, you would need a steel bracket of some sort to make them floating. I have included a link to a company that does something like this, maybe you can give them a jingle and pick their brains?

Good luck!

Here is a link that might be useful: Atlas East

You may find some options at mockett.

Here is a link that might be useful: mockett

Thank you! You've given me some really good options--please keep 'em coming. I'm not sure which ones are viable so the more choices we have, the more likely one will work for us.

I had gotten some floating shelves at IKEA a few yrs. ago and they have a metal bracket tha inserts into the shelf making it quite sturdy. I was pleasantly surprised.

I agree that how you chose to hang it would determined on the style of the buffet in accordance w/ the rest of the style of decr.