Use cases for the TML widget in Vuforia

Seconded

Looking forward to documentation.

That;s correct.  the tml wiget can be use to inject any tml into the scene.  View is a web browser and Tml is the markup language used to define the 3D AR experience.   Declaring shaders is just one of many uses; probably one of the more advanced, as the shader mechanism itself is generally reserved to cgi professionals who understand how to program GPUs!!   Other tml usecaes e.g. defining trackers, or declaring scene content, these are all possible  Real examples are now being created -

Apparently, there is an example of using TML to dynamically highlight parts in the Help Center

Not sure, for how long it has been there, but I haven't seen it.

http://support.ptc.com/help/vuforia/studio/en/#page/Studio_Help_Center%2Fmetadata%2FMetadata_201_Highlight_Parts.html%23

$scope.hilite = function (items, hilite) {
 items.forEach(function(item) {
 tml3dRenderer.setProperties(item, hilite === true ? { shader: "green", hidden: false, opacity: 0.9, phantom: false, decal: true }
 : { shader: "Default", hidden: false, opacity: 1.0, phantom: false, decal: false });
 }) //foreach end
} //hilite function end