TL;DR - I don't think Greenlight is what you think it is, nor (if I'm correct) does DF particularly need Greenlight. On the other hand, it might be interesting to see DF on Steam, but there are a few issues to work through.
I'm pretty sure that Greenlight is more of a way for Valve to crowdsource the decision about which games they should distribute and which they should not, not as actual distribution of Indie games. If Toady One really wanted to release Dwarf Fortress via Steam, I think that it's a well enough known name that Valve would be open to the idea.
The problem though (as it always seems to be) is money. Consider these facts:
- Toady One makes all of his money off of donations. According to him, DF is and always will be free.
- There are Free to Play games on Steam, but all of them have some alternate monetization scheme (none of which are donations).
- Valve/Steam would take a cut of any "donations" to DF (you can't expect them to distribute the game for free).
So basically, Steam would have to implement a donation based payment model and Toady One would have to accept giving up part of his profits to Steam (granted, probably a small part). In exchange, DF would gain the exposure to Steam's user base. I'm not 100% sure that's actually a good thing, but at least it would bring in new users for Toady One, at least in the short run.
One possibility would be to release through Steam and make use of their update mechanism for users while at the same continuing to release the game on his own website. As far as I know, Steam generally doesn't ask for exclusivity on games they distribute, so it's at least possible. Although I'm sure there would have to be some details worked out between Toady One and Steam there, I'm not sure what that would entail.