For your LD trip, are you in a sleeper, or in coach?
If in a sleeper, there is a self-service beverage area in each car which has coffee (maybe?), hot water for supplied tea and hot chocolate, juices, bottled water, and ice.
If in coach, it probably depends on the mood of the snack car attendant, and if you are buying anything else at the time. What you would be doing is like walking into a restaurant and only ordering free hot water because you brought you own stuff. Also, being in coach doesn't mean you are guaranteed to have an electric outlet at your seat to start cooking/heating your own.