The solution is obvious -- have world gen follow the whole tech tree as a tree. It's just a matter of Toady getting around to doing it.
I know that many people here have voiced a dislike for tech trees, calling them too gamey . . . and they are right.
Other people have said that tech trees are the only realistic way to simulate an evolving civilization . . . and
they are right, too.
Unless Toady and ThreeToe truly mean to keep having dwarf civs spring, fully formed, from the forehead of Zeus, then at some point they are going to have to choose.
For now, possible alternatives to checking each civ's (metalworking) tech tree during worldgen are . . .
1) Only place Mountainhomes near sources of iron.
2) If a Mountainhome's territory includes no iron ore, enlarge its borders until iron is "claimed".
3) Once a Mountainhome is placed,
force a vein or two in there.
4) Have all Mountainhome sites check for access to goblinite.
Even if the Mountainhome is located on a tiny island that just
barely happens to include a Mountain biome, option 3 would still give settlers access to iron anvils. Frankly, I find the ability to embark on the opposite corner of the map from one's Mountainhome to be
far less plausible than a civilization that has steel, but not iron. (Their stocks of iron could be really limited, and steel is better for everything, so as soon as they get a little iron, they smelt it into steel. Perfectly reasonable.)