Are these "known keywords" you refer to the key words that are entered in the drop down "Search Engine Optimisation, and Tagging"

How will users know what a keyword is? If the text search is based on predictive search returns of key words in the "Search Engine Optimisation, and Tagging" section I assume as you type "A" words begging with A come up...

Yes, those keywords.

Yes, as you type.
It wouldn't be first character, it would be after you type a few (once it gets a small enough set of possible matches for it to make a suggestion on that).

Working on this now. I'm adding some extra options to configure the minimum autocomplete length, the maximum number of suggestions, and how many times a search must have been done to come up as an autocomplete (for the search-based autocomplete, not the keyword/title autocomplete). I'm also adding lots of privileges for this so you can control what content types it works for, to prevent private content title/keyword leaks if not desirable.