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Well, scripting is a possibility. In Structure Synth I added a JavaScript interpreter, which could be used to script animations (there is a builtin JavaScript compiler in Qt) - but I don't think it the best way to do it. It would be nice to have a scripting language with the same syntax as GLSL (vec3, matrices, etc.). You could probably do something clever with Clang and the GLM library, but it is a lot of work.

Also remember that it is possible to do calculations in the vertex shader, and pass them to the pixel shader. (Though the performance gain is not always great - I guess the few (four) vertice calculations are not enough to hide the GPU latencies. It might, however, reduce the register pressure in the aubsequent pixel shader).

But... first I need to do some work on the keyframe interpolations :-)

Yeah after thinking about this the vertex solution seemed much simpler - maybe you could make a #providesVertexInit (if there isn't one already) and allow the user to write out the varying variables once rather than rewriting the whole vertex shader and then copying the varying definitions to the fragment shader. Its a great program now so don't think of this as badgering
One thing I noticed is I couldn't send as many varying floats as uniform floats - don't know why they have different limits.

Bulatov.frag, limit_set.frag don't runs on Intel HD4600  Both scripts compiles without errors or warnings, but I see just green background 

Updated link to current real time version ShapeJS 
https://shapejs.shapeways.com/v2/examples

I've been just learning the pixel shaders when I wrote it.
I didn't know what I was doing.