a. Don't use a combo box. It's a kind of text field. You want a kind of menu, so use NSPopUpButton.
b. Don't make the control the accessory view. Create a custom view which contains a (weak) reference to the panel, and the control as a subview (possibly with label text to explain, if warranted). Implement the action methods for your menu choices to change the panel's canChooseDirectories property.
It's a while since I've messed around with accessory views, but I think the panel will react to the change of canChooseDirectories property values.