Don't know about large things or plywood, but here is something that I do.
I keep a mason jar filled with scrap iron - old steel wool, old nails etc - nice and rusty. Then I fill the jar with white distilled vinegar and let it sit for a while.
I will then filter the vinegar solution into another jar, and when you soak oak in it, it will stain the oak black - like ebony. Never tried this with a large thing like a desk - but you might give it a try. It is not a surface treatment, it is a stain, and it goes quite deep into the wood, so you can sand and finish like ordinary oak.
Mark