Hi Rhenium!
I have done such reactions, but only for analytical purposes. You can oxidise lignin with nitrobenzene/aq. NaOH and produce a mixture of substituted benzaldehydes that way. Most refs detailing this procedure use a closed reaction vessel, meaning high temp and pressure. It can be done in a glass flask without high pressure, but the reaction rate will be very low. Good enough for analytical work when all you want to know is the ratio of the different benzaldehydes in the lignin, but not quite good enough for production.
One way to solve this dilemma might be using high-boiling solvents instead of aq. NaOH, I have seen aniline mentioned in one ref. But aniline is very toxic and hardly OTC so I can't recomend this variation.
Another big problem is finding the right lignin high in syringyl units. Separating the aldehydes will be very difficult, unless you use chromatography, which can't be used for production quantities.
I'll try to find some refs for this.