I seem to have found a solution that may benefit future newbies, although it hasn't been fully verified yet.
I iterated through all nearby coordinates (a nested for-loop, note: coordinates separated by the resolution, not actual leaf nodes!) and if the (non-existent) leaf node at these coordinates is "unknown", I use setNodeValue(x,y,z,getClampingThresMaxLog(), false) if appropriate (coords match plane). When false (lazyEval) is used, make sure to update the tree using updateInnerOccupancy() afterwards. With true, this is supposedly not necessary.