Наконец-то получил ответ:
@dbus.service.method(dbus.PROPERTIES_IFACE, in_signature='ss', out_signature='v')
def Get(self, interface, prop):
...
@dbus.service.method(dbus.PROPERTIES_IFACE, in_signature='ssv')
def Set(self, interface, prop, value):
...
@dbus.service.method(dbus.PROPERTIES_IFACE, in_signature='s', out_signature='a{sv}')
def GetAll(self, interface):
...