Having a new param is not really an option as the interface of SPARK may be enough as it is to achieve your effect.
You definitely have to write a modifier. The real strength of modifiers is that they can hold their own data per particle and process it.
If you want to interpolate custom data within a modifier, it should be possible, with little modification to SPARK code, to allow modifiers to encapsulates interpolators to interpolate their custom data. I will work on this to see if it is possible with little effort.
Regarding the VectorInterpolator, this can be useful. The only problem is that the way to interpolate the vector depends on what it represents.
For vectors representing positions, a simple linear interpolation on every member whereas for vectors representing a direction, they have to be converted to quaternions and the interpolation is performed on the quaternions, the the result is converted back to a vector.
If you started a modifier and dont manage to get the desire effect, dont hesitate to show you code, so we can help.
@Rampollo : When you talking about integrating 3D textures, you mean in the Direct3D renderer ? Because 3D textures are already supported with OpenGL. If you get a result, I m interested in integrating your code to the official release.