There is a big European-wide timetable change on 10th December. Most tickets after that date are not yet available except Eurostar and SNCF.
Interrail/Eurail have some extra limitations so they're not offering passholder reservations either. However you can use this website of the Belgian Railways (4€ fee per order compared to 2€ per person per train on eurail.com) : https://www.b-europe.com/EN/Booking/Pass#TravelWish
I'd recommend booking Eurostar quite in advance around Christmas, especially if you need to take a specific train. Most trains are still available but the route is really popular and there's a passholder quota... :)
(Another tip : it is cheaper to avoid eurail.com ti book seat reservations mainly due to extra fees but also unreliable info. Don't hesitate to come back for further questions.)
https://www.seat61.com/interrail-and-eurail-reservations.htm
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105