I've done this sort of type-ahead done with Domino using AJAX and a Domino servlet but only for values which are being looked up against a sorted view. And from memory the servlet was "intelligently" caching the data where possible.
The concern with trying to implement this type of search against a full text index would be the speed of the search itself and it's ability to keep up with the typing of the user. I think that FTI searches would struggle to keep up. But I'm happy to be shown otherwise.