Hi, an interesting discussion! As far as I understand the architecture of GP a little bit more now:
- sending MIDI over an MIDI out block within an
On Deactivateis not possible, as all MIDI blocks get deactivated before the callback gets called - some processes/function are asynchronus (I already made some more experiences while the implementation of my latest version of my Arturia KeyLab Scriptlet …)
However I finally have have been able to send OSC messages to the GP listening port out of my Scriptlet. Funny side note: moving some sliders of an open scriptlet panel may even send further OSC messages, even if the Rackspace where this Scriptlet resides is not active 
In my Global Rackspace I’ve implemented a callback for my OSC message and can react on.
For me everything is fine at the moment. Thanks for pointing me in the right direction @dhj 
- and all others
Some details of my solution:
Reading out the used OSC Listener Port in On Activate and injecting an OSC message in On Deactivate (OSC_ListeningPort is a global integer…)
Catching the OSC message in the Global Rackspace script:
More details in the linked thread 