Not sure I understand what you’re trying to do, though. Just because Cesium is fixed to WGS84 doesn’t mean your visualisation of stuff is? For most things, convert from coord system X to WGS84, visualise it, and if any of the metadata needs conversion, then do that back to coord system X. Remember that the base datum of a whole 3D system is easier to optimize to a fixed model than a dynamic one, and I like that it’s a fixed model so that we don’t get into the millions of things that could go wrong with a mixed content coord system / model.
But also as stated, you can create a datum that’s different from WGS84, but as soon as you venture into rotation and projection, you’re in for a world of pain. It’s simply not worth doing. So, if I was you, I’d convert stuff from X to WGS84, visualize it, and any metadata (for either processing or visualising) convert it back to X (things like surface coords, coords on a model, or transformations, or whatever). PROJ4 (which I now think is PROJ?) is fast enough not to worry about it, even though it will require some effort on your part as it’s not out of the box.