You can use the undocumented selectednodes attribute, which was added in Softimage 2014.

views = Application.Desktop.ActiveLayout.Views
v = views.Find("Fx Tree") or views.Find("View Manager").Views.Find("Fx Tree")
if v:
    print v.GetAttributeValue("selectednodes")  # None

The FxTree view also has a targetcontent attribute, which is documented.

* hat tips to csaez (code) and luceric (attribute)