Ref T12314. If you set a form to have the "plant" subtype, then create a task with it, save "plant" as the task subtype.
For Conduit, the default subtype is used by default, but a new "subtype" transaction is exposed. You can apply this transaction at create time to create an object of a certain subtype, or at any later time to change the subtype of an object.
This still doesn't do anything particularly useful or interesting.