If I book ahead for our upcoming train trip (Ottawa-Montreal-Quebec City and back), is there a chance of being assigned seats that face backward? I'm prone to motion sickness and must face in the direction of motion.

I'm going to phone the toll-free VIA # and ask them. But thanks anyway!

I am the same way, Judy! Makes me nauseous just thinking about sitting "the wrong way".
Did the give you the seat you want?

Are you going this weekend (July 22-24)?

Ocean

Most Via seats face forward but they do have some set up for groups of four. The odds are greatly in your favour. Some trains do not have assigned seating. The conductors are good at fixing this sort of issue.

I've taken the VIA train quite often and recently (May) between Ottawa and Montreal, and Montreal to Toronto. Gavin's comments are correct, and there were no seat assignment if you board the train at Ottawa or Montreal during non peak hours. If you board at any stops between those cities, you could be assigned seats, but 99% of the seats face the way you want anyway.
I also see no problem in them being able to accommodate you if you specifically request for the right facing seats as the trains are rarely that full. Don't know about Quebec City but imagine its much the same.