Do you need to use the Material Picker extension? If not, you could change materials in the scene with JavaScript using the viewer API, see: API change material for another - #2 by jan
var dark = viewer.findMaterial('Flooring-Dark');
var light = viewer.findMeshesWithMaterial('Flooring-light');
var med = viewer.findMeshesWithMaterial('Flooring-Med');
for (var i = 0; i < light.length; ++i) {
viewer.setMaterialForMesh(dark, light[i]);
}
for (var i = 0; i < med.length; ++i) {
viewer.setMaterialForMesh(dark, med[i]);
}
viewer.requestFrame();