Use Okular / Amarok's passive popup class to display things like "13 replacements done" or "Document successfully loaded". The needed class is PageViewMessage in the files pageviewutils.cpp and pageview.h. Maybe it is a good idea to alter the function slightly from
void display( const QString & message, const QString & details = QString(), Icon icon = Info, int durationMs = 4000 );
void display( const QString & message, const QString & details = QString(), Icon icon = Info, Qt::Corner corner = Qt::TopLeft, durationMs = 4000 );
Maybe KPassivePopup works, too. It can show arbitrary widgets as content. But it's more ugly.
KateDocManager derives from QStandardItemModel. Since the view is a plugin now, this should be converted back to a simple list.