Hmmm... so the biome switching might work.
Since the biome is geographical region dependant... you could switch the biomes for any part of a region you are embarking into. You could end up have a microcosm region (you can have up to 5-6 geographical regions, right?). It would be cool to have a little bit of freshwater ocean, little desert, little tundra, and some mountains in the same embark. Now that I think about it though... the heights and climates for everything have already been determined... you would only be changing the plant and animal life, right? So if you embarked a desert and changed it into a mountain biome, you would find yourself in a desert populated by mountain creatures. If you really wanted to do a mountain though... it would still be possible, but the biome tag isn't the only thing you need. You would need to go ahead and make a complete editor for the height of each unit in the embark map.
Now that I think about it... you really would need to make a full map editor to get mountain regions working by editing a different region. However, the converse is not true; if toady has all mountain biomes disabled for settlement... just change the biome tag for the mountain region and you should be able to settle there.
If you really wanted to find your problem though... start with a desert and start splicing in code from a mountain region until embark is disabled.
[ March 25, 2008: Message edited by: Maher ]