Ciao Sascha,
I am planing to set up a experimantal plot with Sap analysis the upcomimg season so I could try if we can improve total nitrogen in musts.
As we also manufacture natural wines I made the same observations as you did. In the last dry years we also had problems with volatile acidity and slowed down, even stopping fermentation due to low nitrogen contents. This year was way better. Although we had trouble keeping up with Oidium and Peronospora with this wet season the allover fermentation performance is much better due to less drought stress.
We work with biodynamic practises and also selfmade ferments for foliar and soil application to increade soil organic matter, but it works quite slow. So I hope to get some gut analysis results in the next year.