Currently there is no way to specify a start path for a an existing tree.
Path has to be set up in custom tree configuration java file which makes trees less reusable.

For example, if I needed a website tree in a dialog link controll that starts from /site1/folder1/ I would have to make new custom tree configuration that extends website tree and sets the path.
Instead, it would be nice if I could just make a copy of the website tree configuration node and specify path = /site1/folder1.