Yes, I have tried to do it as shown in this example.
I am thinking that another relevant example is this one:
https://kitware.github.io/vtk-js/examples/ScalarToRGBA.html
However, I am still not clear how to “supply” scalar data associated with points. The examples with PlaneSource use formula filters and such to specify the numerical data, and my objective is to read that information as an array from an external source.