I managed to find an acceptable translation for Ethereals: "Etheriƫrs". And an Ethereal soldier is still "Etherische soldaat", which actually sounds like it could be a true Dutch word.
Unfortunately, I was unable to coax The Gimp into opening the Dutch.geo image file in a way that it was recognisable, so this is still the original.
Anyway, I've made a pull request (
https://github.com/SupSuper/OpenXcom/pull/629), so hopefully we can all play the game in Dutch soon!