but generally you can pass any property tree node to a shader via an uniform, even FDM-driven ones.
Nope - at the moment JSBSim created tied properties which the effect framework doesn't handle gracefully, which is why you can see the Shuttle pass a lot of FDM stuff through trivial gain filters just to make the output palatable for the shaders. Not sure what YaSim does.
In a broader sense, the issue is how much of this we'd like to hard-code and how much have configurable.
Think e.g. of a quad-copter drone - do we want to separate the downwash into four flow fields or merge it into one? The effect framework will (obviously) take only a limited set of nodes, so should YaSim really get to decide what to do for more than one rotor?
I'm very reluctant to hard-code such things and take them out of control of the maintainer, the outcome might easily be poor.