The problem why it was not working earlier, was that it segfaulted on every creation of KDE classes. That's something I really hate about C/C++: the lack of stack traces, though valgrind was helpful. It turned out that adding the below line fixed all. A KInstance is needed when using KDE technology outside a KDE program:
KInstance instance( "strigita_kfile" );
Combine this with the xattr support added by Jos earlier today, I hope to see an interesting new Strigi release soon! Now we only need to get editing of keywords into KDE4.