The values for the texture are drawn from the volume voxel values, not from the rendering. If you convert the segmentation to a labelmap volume and probe that, the label values would be on the mesh (although I suspect there is interpolation involved, which might result in undesired fractional values of labels near boundaries, and it would be label values and not color values which would be stored). I don’t know if you could probe an RGB volume and get out all three scalar values using “Probe volume with model”. It looks like that may be within the capabilities of vtkProbeFilter, but was probably not the imagined use case when the Slicer module was being put together.
Correct, volume rendering transfer functions do not affect probing. If you probe a scalar volume then you can apply transfer functions on the values by a couple of lines of Python code (get the array and run each value through the color transfer function).
We played with this a bit on RGB volumes (cryosection data) and the result was pretty disappointing. It was not surprising, because segment surfaces tend to have fairly uniform color (non-uniformities are usually under the surface). Therefore, we did not continue exploring “saving volume rendering as colored surface”.
If you have examples of surface rendering beautifully displaying volume rendering results then we can revisit this. Can you share a few examples that shows the volume rendering and colored surface rendering?