The function handle_set_active_port in src/modules/dbus/iface-device.c requires the requested port to be submitted as a path of the form /org/pulseaudio/core1/sink0/port0, which is consistent with what handle_get_active_port returns.  However inside in that function there is a call 

if (!(new_active = pa_hashmap_get(d->ports, new_active_path))) {

but the d->ports is indexed by the name of the device, not the path, so it cannot find a matching port.  As you can't pass the name of the device as an argument, it is therefore not possible to select the port over dbus.