There are 2 potential uses of ultrasound in wood processing, namely, assist/improve preservation and drying or monitor sawmilling and drying.

Using ultrasound to improve liquid penetration into wood and also improve its kiln drying has been studied in few occasions in the past. The idea is behind ultrasonic cavitation or the formation of millions of mini bubbles that burst thus raising pressure and heat at micro-scale (ultrasonic cleaning of glassware in chemistry labs is a good example of current application).

So, in the case of wood, this combined phenomenon is thought to open up its internal porous structure thus, allowing easier flow in and out of fluids - preservatives in impregnation or water in drying. For example, under ultrasonic blast, pressure treatment of wood would result in better and deeper penetration in difficult to treat species. The reverse will be applicable to removing liquids from wood, aka, drying. 

Now, the trick here is the requirement of a liquid medium present between the ultrasound producing piece of equipment (called "the horn") and the piece of wood. This liquid (coupling medium) has to be there to allow the transfer of the ultrasonic waves to wood from the horn. In pressure treatments, it is always there, so no problem with its application. However, in drying, this idea is quite difficult to implement due to the lack of that coupling medium. It works nicely in the lab with small specimens, but inside a commercial dry kiln, as you can imaging, it is not easy to use.

Ultrasound has also been used as a monitoring non-destructive way for wood cutting and lumber drying. In both cases, these operations produce sound and by isolating frequencies especially, at the ultrasound part of the spectrum, you can "listen" to the operation and when you exceed a particular level of output that is determined through experimentation, you change the speed of the process. For example, in drying, when you remove water fast, you may shrink the wood too fast in high temperatures and thus produce a high number of micro-cracking of the cell-wall that will emit sound. If you listen to this sound and you have proven that above a particular level, too much honeycomb is produced, then you just slow down by either reducing temperature or increasing humidity. Like you go easy on the throttle of your car when strange noises come from your engine above a fixed number of RPMs. Same idea applies to sawing when monitoring the wood cutting noise.

Again here, we have to deal with good coupling between the wood and the detector (opposite of the horn in the case) plus answering the question of "how many pieces of lumber are you going to monitor in a commercial kiln?". Great idea that works nicely at a bench-top or even pilot scale level, but when it comes to use it in a commercial kiln, background noise, vibrations and the number of lumber pieces you got to monitor in order to have a good idea of micro-cracking becomes quite challenging.

I'm sure that others can add more here, but this is in a nut-shell the status of ultrasound application to wood processing I could think off the top of my head on a Monday morning.