By the time your fortress reached year 4000 you'd have probably worked out a way to make some kind of automobile thanks to exploits in the game code coupled with existing machinery... some kind of water powered perpetual motion cavein, falling constantly and never hitting the ground. or something... after all, how many gameplay hours is that?
Anyway, the idea of inventions in worldgen is a solid winner. It's a fantastic idea, seriously. I say make all workshops and labours available from creation, but all smeltings beyond a few very easy ones are discoverable, as are nearly all creatable items except the most rudimentary (stone axe ftw). Allow certain techs to be tagged "required" by users in RAW so if you don't want to play in a world that doesn't have steel, you can ensure that the world is rejected unless steel was discovered.
Tech theft would be mediocre. I mean, humans trading steel off dwarves doesn't mean they could figure out how to make it. It's a complicated process. Likewise, owning an elven wooden harp doesn't make a dwarven craftsman capable of making one out of stone.
regarding inventive "moods" for legendary dwarves, 2 thoughts:
1) only dwarves who'd reached Legendary and still had no artifact moods should qualify, making these moods very rare, and
2) chances should be much higher (50:1 or so) that the invented item will be something another race has discovered and the fortress has obtained by trade or some other method. This means Urist is more likely to figure out how to make a dwarven stone harp by retroengineering the elven wood harp than she is to invent a rock guitar (hyuck hyuck) entirely from scratch.