Will Cesium support bathymetric terrain where the elevations are mostly less than 0? Looking at the proxy.java code (https://github.com/AnalyticalGraphicsInc/cesium/blob/e1fc729ea2401d1e326dca85ac106baa0752e64f/Tools/proxy/src/com/agi/TerrainTranscodingHandler.java) and the Southhampton cesium-terrain-builder code it appears that the elevations will only go down to -1000 meters.
I believe Cesium should support this. The code you posted isn’t part of the Cesium code base anymore.
The Cesium terrain heightmap format, as defined at https://cesiumjs.org/data-and-assets/terrain/formats/heightmap-1.0.html , does indeed have a limitation of -1000 meters. The quantized-mesh format, at https://cesiumjs.org/data-and-assets/terrain/formats/quantized-mesh-1.0.html , looks like it might not have that limitation.