Traveling Amsterdam-Brussels with KLM ticket on Thalys - When / where does my ticket get stamped?

I purchased a ticket to Brussels via Amsterdam on KLM, with the Schiphol to Brussels-Midi portion on the Thalys. The return portion of my itinerary is on Air France several days later. This was done in good faith (i.e., not as a fare dodge) since I really do need to go to Brussels.

However, it now turns out that I have a meeting in Amsterdam the day I arrive.

I realise I am required to board the train to Brussels to fulfill the terms of my KLM ticket, and that I will need to get my ticket stamped on the Thalys in order to board my return flight. What I want to know is where and when this takes place, since I need to return to Amsterdam. Will I need to ride the train all the way to Brussels, or can I get my ticket stamped and then disembark in Rotterdam or Antwerp?

Alternatively, is there any possibility of getting my KLM Thalys ticket issued for a later train, so that I can attend my meeting in Amsterdam before heading to Brussels?

NOTE: Some users have marked this question as duplicate. Be assured that I read other Thalys/KLM questions others carefully prior to posting, but none of them address my key question: Where/when during the Thalys ride does the ticket get stamped? This is important to anyone in my situation -- i.e., needing to return to Amsterdam after boarding the Thalys, but needing their ticket stamped to ensure their return itinerary is not cancelled.