Actually you can type into the sub-folder text box when “Custom Folder” selected.
So, on further testing, although things have changed a bit I am still able to accomplish what I want so no longer a showstopper for me.
One small bug: when you hover the mouse over the Path field it turns into a text entry icon which indicates you should be able to enter text but you actually cannot! DxO should just allow us to enter text into the Path field and all will be well. I guess they don’t want to validate that the Path actually exists before exporting as you are forced to select a folder through a folder select dialog or selecting the folder your file to export resides in, negating the need to verify the existence of the selected folder.