Looks like there is no native Windows dialog that offers that functionality.
There are indeed some that seem to use the SaveFileDialog with a dummy filename, but that is not really an elegant solution.
$PickFolder=New-Object-Typename System.Windows.Forms.OpenFileDialog
$PickFolder.FileName='dummy'
$PickFolder.Filter='Folder Selection|*.*'
$PickFolder.AddExtension=$false
$PickFolder.CheckFileExists=$false
$PickFolder.Multiselect=$false
$PickFolder.CheckPathExists=$true
$PickFolder.ShowReadOnly=$false
$PickFolder.ReadOnlyChecked=$true
$PickFolder.ValidateNames=$false
$PickFolder.ShowDialog()
Split-Path$PickFolder.FileName