Thanks for the link to the VividTree.
But in the meanwhile I found what I was looking for.
I extended my customdraw method of my TreeCtrl with the ITEMPOSTPAINT drawing stage to draw the border around the items I want to additionally highlight when the TreeCtrl does not have the focus:
case CDDS_ITEMPOSTPAINT:
if((tvcdPtr->nmcd.uItemState & CDIS_SELECTED)
&& (GetFocus() != this))
{
RECT r;
CreatePen(PS_SOLID, 1, RGB(0, 0, 0));
SelectObject(tvcdPtr->nmcd.hdc, GetStockObject(HOLLOW_BRUSH));
HTREEITEM hItem = GetSelectedItem();
GetItemRect(hItem, &r, TRUE);
Rectangle(tvcdPtr->nmcd.hdc, r.left, r.top, r.right, r.bottom);
}
break;