Question for SS 3.*
When you use dataobjects as pages for a section of your website, is it possible to have these objects listed in TreeDropdownField?
I doubt it. I think you can only pass one class to TreeDropdownField, and it needs to implement Hierarchy. Can you change your design to use Pages as Pages instead of Dataobjects as Pages? 
I certainly can and have already decided to do so. This problem left me wondering however since using dataobjects as pages is quite common in SilverStripe projects (even taught in the tutorials by UncleCheese) yet the inability to include them in TreeDropdownField takes away the possibility to create links to such pages in the CMS.
Yeah I think the ‘dataobjects as pages’ idea can be useful, particularly if the datamodel is used elsewhere, but generally speaking if you want a dataobject to behave like a page then it’s probably best to just make it a page in the first place.