Transparent overlay on Mesh

I am interested in overlaying a heat-map over a mesh in Unity3D with some transparency such that it would be possible to see the texture underneath. I cannot pre-make the heat map offline because it is dependant on user input.

It seems that since the "heat map" will use multiple different colors I cannot use a line renderer or trail renderer without created one per color.

It seems I cannot create a world-space GUI because I want the "heat-map" to fit the height of the underlying mesh. Unless there is someway of fitting a canvas to a mesh?

As far as I can tell the only approach is to use multiple materials on the mesh using the second material as the heat-map which I can control the opacity of. I can then call setPixel on the 2nd material to modify the color/opacity as I see fit.

Am I missing an easier solution?

Thanks for any help.