I still believe it's possible to do it with a good heuristic. Vim once had a
plugin for code completion called icomplete [1]. It's only based on
exuberant-ctags, afaik that's what you use as well.
There also is a IDE for Qt development called QDevelop [2]. QDevelop portet
icomplete to Qt and named it qicomplete (you can find it in QDevelop's
source code). Maybe we can borrow ideas from qicomplete, or maybe even port
it to Kate. It should work in >90% of the cases as far as I know, which
would be really good (good enough for sure) for Kate.

Cheers,
Dominik

[1] http://stud4.tuwien.ac.at/~e0125672/icomplete/
[2] http://qdevelop.free.fr/