I cannot imagine that sanding and buffing a wooden spoon would have any useful culinary applications, although I suppose it might feel smoother to the touch.
Generally when you season something wooden for food preparation, the purpose is to create a protective layer to avoid warping or impregnation.
If I really wanted to season a spoon, I would season it the same way as a wooden cutting board: Use a food-grade mineral oil and rub it in thoroughly with a cloth or paper towel, then repeat as necessary after it is dry.
Like hobodave, however, I have never done this nor heard of it being done. The cost of a wooden spoon is probably less than the cost of the mineral oil you'd need to season one. If your spoon starts to warp or take on strange odours, just buy a new one.
N.B. I can think of one exception, which is if the spoon takes on a fuzzy texture when you get it wet. In that case, sanding it would be done in the context of raising the grain. But once again, it's extremely rare that you'd need to do this with a cooking utensil - this is a far more common requirement for homemade (i.e. DIY) wood products.
I would recommend using Whiskey Stones. They are used in whiskey instead of ice cubes. So they should be heavy, won't rust, and are supposed to be immersed in liquid that you'll consume. I think that probably meets all your criteria.
Whiskey Stones
Another alternative is to use a rack. This comes with the Sous Vide Supreme and I find it quite useful for keeping meat submerged. It does however only accommodate certain sizes of meat. It works quite well. You can buy it separately online for $12.
Best Answer
There has actually been some significant research into cutlery being made of the correct substance to match the food
Testing shows perceptible difference in tastes from various metals when combined with certain foods. Just as silver spoons and egg are an unpleasant experience, some metals (e.g. copper, zinc) acted as a catalyst and improved the taste experience of certain foods
We use wood and ceramic for similar reasons, often for the mouth feel of the cutlery and heat dispersion, e.g. hot soups with metal spoons can be painful
Glass would have it's place, but probably not to dissimilar to porcelain (ceramic). Glass spoons are available, but rare
Some references:
Institute of Making - Sensoaesthetic Properties of Materials
Fine Dining Lovers - Cutlery food science