Could you tell me your conjectures, please!? Eventually I can try to help with further information.
An idea of mine: as of with network-manager-gnome the notifications in kde have the proper icons, you could ask them how they achieve that. Could it be because of hplip doesn't install its icons into /usr/share/icons/hicolor which is the default place, I think? Correct me, if I'm wrong.
denk