There’s nothing out-of-the-box in Cesium that will do what you want. It’s possible by writing some new code, of course.
As an experiment, I hacked up Cesium to render the entire terrain surface with an alpha value of 0.7. You can see my experiment in the translucentTerrain branch:
Looking at that may give you a sense of some of the problems you’ll face. One problem is that regular alpha blending of translucent surfaces requires that the surfaces be rendered in back-to-front order, which is opposite the order that we usually render terrain. It may be possible to leverage the work in the order-independent translucency (oit) branch to get correct translucency without sorting. I’m not sure offhand what the costs would be for terrain rendering.
Another problem is that translucent terrain reveals the skirts that drop down from the edges of terrain tiles to hide small differences between adjacent levels-of-detail. If the translucent surface has a relatively low alpha value, it may be reasonable to simply skip rendering the skirts. Other, more complicated things are possible.
Beyond that, you have the problem of actually rendering two sets of geometry within the pushed region. One approach is to simply make the existing terrain translucent within the pushed region instead of actually pushing it. Then use a gltf model or something similar to render the underground geometry. Again you have render order issues with the translucency, or you need to use order-independent techniques.
It’s also possible, with some more work, to render two terrain surfaces. We took some very small steps toward that in the tiledGeometry branch:
This probably isn’t the easy answer you were hoping for, but I hope it helps!