In general there is no such thing as a "pH of wood" - at least not in a direct sense, just like there is no such thing as a "pH of a soil".
What typical procedures do is they ask for mixing the solid with water and measuring pH of the sludge. Trick is procedure has to be well described (amounts and things like waiting times, pH electrode placement etc.) and followed, to make results comparable. I am not aware of an established procedure accepted as the "right one", could be the wood industry has something like that (can depend on the country you are in though).