Right now, the conversion will need to happen outside of CesiumJS.
As you mentioned, converting between reference systems varies by location. If we implemented this conversion in CesiumJS, we’d want to avoid bloating the library, so most likely we’d rely on an external API requests to perform the conversion.
We have support for MSL heights documented in KML Graphics Roadmap · Issue #2179 · CesiumGS/cesium · GitHub, and I’ll bump that thread with this use case.