I have not come across a directory selection widget, so have used the standard file selection and instruct the user to open the required folder then push the appropriate apply button. The actual file selection box is not used/ignored. Deliberately truncating the size of the file widget to hide this detail and adding a button manually to do the apply can be done, though I'm not certain how well this size truncation would work across different environments.
Attached is an example form to illustrate what I mean by using the vertical sizing to remove/hide detail off the bottom of the file widget. It is not complete - just enough to illustrate what I mean. It may be you can do something similar that makes sense for you application.
