IBus 1.5.10 was released recently.
It can show the IBus panel icon in KDE5. KDE5 no longer enables notification area by default so this version communicates with KNotification via DBus instead of GtkStatusIcon.
KDE5 is available in Fedora 22 or Ubuntu 14.10 and IBus 1.5.10 is integrated in Fedora 22.
The IBus panel checks the environment value XDG_CURRENT_DESKTOP which is set by the login manager likes GDM.
If XDG_CURRENT_DESKTOP=KDE, ibus panel shows the ibus icon connecting KNotification.
If not, ibus panel shows the ibus icon with GtkStatusIcon.
Then if you run startkde on xterm, you need to set XDG_CURRENT_DESKTOP=KDE before you launch KDE5.
If you use KDE4, the configuration can be disabled in the ibus build time and ibus always enables GtkStatusIcon. E.g.
# ./configure --disable-appindicator --prefix=/usr # make
There are two known problems in IBus Indicator for KDE5 plasma-desktop.
One is that KNotification requires icon theme directories to show the icons. Many ibus engines have the custom paths for their icons and send the full path to GtkStatusIcon. If their engines wish the custom paths, the paths need to be icon theme paths. Probably it would be easy to move the icons to the standard theme paths, e.g. /usr/share/icons/hicolor. ibus-anthy is already done.
Another is KNotification does not support to draw the string icon under GTK3. It needs to send QIcon or something so currently IBus XKB engines always show ibus-keyboard icon instead of the strings by layouts.
I filed https://bugs.launchpad.net/bugs/1403451
Maybe KNotification needs to convert a string to QIcon or IBus panel needs to load QT libraries.
IBus 1.5.10 also has the feature to hide IBus engines on gnome-shell 3.16 lock screen so that preedit text is not shown in password.