I have just dabbled with Houdini and what you say (it is not exactly a shader but its procedural interface is probably similar to what you are saying) is spot-o

I have just dabbled with Houdini and what you say (it is not exactly a shader but its procedural interface is probably similar to what you are saying) is spot-on. Lots of data incoming, a graph of operations, lots of data (and plots, and what not) outgoing.