I discovered that KMenu::addTitle can crash the calling application on Mac OS X when the receiving menu is in the (global, toplevel) menubar. The crash happens somewhere deep in Qt, and doesn't occur when adding a title to a popup menu for instance, so the issue must be with Qt and not KDE.
I'm looking into emulating the feature at the level of the calling application (because I don't see how to determine in KMenu::addTitle what kind of menu we're adding to). One possibility preserving the title text would be to add an inactive (grayed out) regular menu.

Being quite new to KDE development, what would be the best, leanest way to do so?

NB: a prime victim of this issue is kmail.