Oh I see. Sorry, I misunderstood your question (well, actually I am curious of how Snips uses the customData field anyway )
I believe that concern is left to the message sender.
From Snips Dialogue API reference:
Additional information that can be provided by the handler. Each message related to the new session - sent by the Dialogue Manager - will contain this data
The customData field is forwarded for message sent by the Dialogue Manager, in this case only SessionStarted, SessionQueued, SessionEnded. In that case customData is filled in (at least judging from the source code). But customData has to be filled in the StartSession message, which is sent by the wake word component IIRC. What you can do however, is update customData when you send the continueSession message. That will be stored and forwarded by Rhasspy later.
What I mean is Rhasspy just reacts to dialogue manager messages and acts accordingly. But I’m just a contributor, Rhasspy core developers could help in better figuring this out.