I ended up calling the minimal representation to create a compact heighfield "lean heighfield". I'm not sure if it is particularly good name, but at least it is more distinct than "compressible heighfield".
If you were previously regenerating tiles using Recast at runtime, this modification allows you to save and compress the voxelized results and speed up the tile rebuilding almost by an order of magnitude (it usually varies between 5-10 times faster).
I will keep on prototyping some more to see if I can support modifying the Detour meshes directly.
In the process I also changed the navmesh builder, so that the detail mesh is now optional. Detail mesh is important if you have large tiles or one big navmesh, or if you require some approximation of the height at runtime. When using small tiles–which is good practice when you are likely to change them alot at runtime–the detail mesh is not always necessary. Not requiring to the detail mesh speeds up the generation process and requires less memory.
[EDIT] As usual, the Visual Studio project is lagging behind. I will try to allocate a slot on some win machine to put it up to date.