There you go. Sometimes I wonder if Firaxis programmers do some stuff just cause they hate modders
Turns out the "missing" LM Terrains are there, with editable stats and all, you can even add a new graphic. They are just disabled inside the BIQ. After finding the relevant byte and changing one 0 to 1, not only you get a new terrain but you can edit it in the editor...
EDIT: Ok, so it's not a graphics problem, but map placement problem. Namely the editor won't add the new terrain to the map, you have to add the base terrain first (e.g. Tundra), then turn it into a Landmark with HexCmp (see link below for a tutorial).
If you want to try it for yourself, you need to learn the
BIQ structure and hex editing. I've posted a little tutorial in the
Deepwater harbours thread. It shows how to add Sea tiles "under" Coast tiles, but also shows how to use HexCmp in general. For enabling a disabled LM Terrain, you've got to find the second occurence of the terrain name, and move 33 bytes back, then change 00 to 01... that's more or less it
