My connection at Bruxelles-Midi station was delayed. I am in possession of a DB first-class ticket to join an outbound ICE service.
Am I eligible to use the Thalys lounge while I await the connection?
Thalys homepage says that:
You have access to the Thalys Lounges for a pleasant experience before or after your journey.
As I understand this, you cannot use your DB ticket to access the lounge. You only have access to it if you arrive in Brussels on a premium ticket.