He appeared in
Black Madonna, which is
Da Vinci Code but set in the Sixth World and published years before (and it has the Vatican sending nukes).
[ Spoiler ]
In that book, Leonardo hacks the megacorps' systems and blackmail them for money. He also leaves clues so that people clever enough to understand them could find and join him. His goal is to build some kind of kaer to be ready for the Horrors.
The problem was that the authors went too far for the line developper (IIRC it was still Tom Dowd back then) who had to wrote this short story (that I can't seem to find in English) with Lofwyr getting inside Leonardo's place, giving another version of the facts and destroying everything, in order to somehow retcon things.
Leonardo has been somehow able to copy the otaku's ability (he could access the Matrix without a cyberdeck) and the last time I've heard of him, rumor has it he wad working in SK's secret lab on the North Pole.
EDIT : Oh, and his works probably helped build Deus. And if the book implied that he was Leonardo Da Vinci, the short story had Lofwyr say that he wasn't (that he just admires him).