it is not clear whether the German Constitutional Court is in a position to rule against either the EPO or the UPC.
Sure it is. The primary argument revolves around the article of the German Constitution which states that only a German court's decisions have validity over German subjects. This has been interpreted as "court with German representation". ECJ, ECHR, etc are OK as they all have German representation.
UPC fails that tests - its panels can be convened in a way where a country has no representation. That is pretty much end of story - the convention in its current form is a classic case of some IPR lobbies thinking that they are above all law and can invalidate criminal, civil legal code and even constitutions with impunity.
The "Professional Jobsworth" product of Ecole d'Administration is just an icing on the cake.
By the way, I suspect Germany is not the only country in Europe with a constitution clause like this. I am pretty sure that some digging will turn up at least one or more countries to raise a similar court case.