I would think that starting with dry material would be better. Whether doing an A/B, STB, or starting with a solvent like alcohol, being dry allows the solvent or water that carries H+ or OH- to penetrate deeper into the material and thus pulls more alkaloids into solution. When there is water in the material, this has to be displaced by solvent or ions have to penetrate molecular barriers built by the plant.
And to top it off, it's very difficult to pulverize wet material into a powder as the moisture protects the material. Water is pretty much an incompressable fluid which takes most of the blow from a blade or mill head and spreads the force over more material rather than letting the implement bite into or shatter the material. The finer the material is ground, the more surface area is in contact with acid, base, or solvent.
If anyone has ever tried to mow a lawn right after a heavy rain storm, you might have some insight into how it is much easier to process organic matter when dry.
I have not done this yet, but the way I would approach it would be to mulch wet for faster drying. Mulch it again when dry. Then put it in a burr mill (coffee grinder). This will produce a dusty sand texture. Some grain mills might be able to handle it and produce a fine powder but are much more expensive.