There’s another way… I’ve tested it and it works good, but I must perform other tests to check every possibilities.
This was basically what I’ve done:
1 create the basic object, unwrap, add a multi-res and sculpt.
2 prepare a temporary image texture, for baking the original vertices position (preferably with the float option)
3 with the multires turned off, bake the Object coordinates (i.e. plug the object coords to an emit shader)
4 create another texture, to store the displacement (also with float)
5 turn multires on
6 subtract the first bake from the object coordinates and bake.
the result will be the XYZ displacement map.
I’ve notice that we must be carefull to store the textures without any color correction. (save as render, exr or anything that allows negative values, node set to non-color, etc)