I am using S88 MKII as my controller and with the sync option on the Widget properties any change in a widget is reflected back to my S88.
So no need for relative mode.
I know that in the forums some time ago a script was developed to support relative mode, but I am not sure if it is still available.
Found it, but I am not sure if you are searching for the opposite.
What messages are shown in the global MIDI monitor when you move your controller?