dwl, you have some exceptional ideas.
First off, I am not experienced enough to offer any real world opinion, so take this for what it is.
Here are a couple of possible ideas:
1. I was reading something the other day about guys using white glue and paper (even newspaper) to temporarily adhere smaller pieces to larger more manageable pieces. Once you are done, you can simply wet the pieces and seperate them since the glue is not waterproof and the paper between the pieces of wood should keep them from joining each other more permanently.
I've done this same or similar process for carving. You glue newspaper onto MDF then glue to piece to be carved onto the newspaper. When done you take a chisel or whatever, and pry the carving from the newspaper. This works as I've have personal experience doing this. However, the carving can handle the prying. I do not think thin walnut can handle the prying or the water necessary to separate the two members. I have not
done this, but it may be worth a try.
2. If you are planning on adding a thin layer of walnut to the birch piece and then veneering more walnut onto the top of the walnet covered birch, can you sandwich some walnut and birch together then resaw through the walnut then plane both pieces of walnut covered birch to youe desired thickness?
Good idea, but in my case the birch ply is 12"x13" (which represents one side of my jewelry box) and my band saw can only handle 8-1/4". This is why I am adding two sheets of walnut to the birch ply: 8"x13" and 4"x13". This will cover the birch ply.
3. Can you color the birch dark (walnut color) before applying the veneer so the pinholes would not show "white" spots from the birch underneath?
Ah, now you're peeking into my brain. This is actually Plan B. I have some alcohol soluble Dark Walnut dye that may work perfectly for this. However, I really wanted to try and add the walnut layer first, but as we now know, this creates other issues of its own which must be resolved before we can move forward.
And, one more thing: the adhesive I will be using is supposed to dry to a dark color, so this may be all that will be needed.
But, again, I have not done any testing so I cannot be certian. If all goes well tomorrow, I will get some serious testing done and will know a bit more.
(Note. COMED has not been able to keep us in power for most of this past week. This has slowed my progress to a snail's pace and I'm not a bit happy about this at all.)
I hope these thoughts help or at least get you thinking about other (better) more creative ways to get past your problem.
You're a thinker alright and that's a good thing in my book.
Thanks for stopping by to lend a hand.