You like or know Richard Foote?. There is another guy out there, called Marcelo Ochoa and he is building it (= an index =) from scratch via ODCI (The Oracle Data Cartridge Interface). If you want to know how you could build them on your own or how the Lucene Domain Index works based on ODCI, then give him a change during Oracle Open World 2009 and vote for him via Oracle Mix.
More info regarding Marcelo or his session (and please don’t forget to vote for him) can be found in his blogpost: Unleash Oracle ODCI API – OOW09 Voting Session
If Marco says it’s good, it must be good. Voted.