[Answering to my self, but also to the original asker:]

Maybe a more elegant solution for keeping rows expanded by default is by reimplementing the following method:

void QAbstractItemView::rowsInserted ( const QModelIndex & parent, int start, int end )

In the reimplementation, besides the super class effect, explicitly expand all collapsed rows that were inserted [as well as their children that may already be present]...

(However, it would be nice if in a future version of Qt, automatically managing such properties would be present -- like also automatically resizing columns. This of course unless explicitly overruled by the user (e.g. reducing the column with interactively, or collapsing an item interactively.)