Well, I can’t answer the finish question, but will make a comment. To replace wooden spokes, the wood is dried to a very low moisture content. The wheels are then assembled, and the wood absorbs moisture from the air to swell and tighten the wheel. I want to say that it’s dried to about 2% moisture and then gets up to about 15% moisture content, but those may not be the right numbers.
Either way, you don’t want to seal the wood right after the wheels are rebuilt. I was told once that you should wait at least two months before sealing or finishing wood, to let them tighten correctly.
Also, make sure the rebuilder knows you’re going to leave the wood natural rather than paint it. This should influence the wood that he uses, so that he makes cosmetically appealing spokes.