Have you checked the Eurostar to Amsterdam and/or Lille? Both those cities have  connections to Brussels.

17 dec:

Brussels: 1st class seats available on 19.34

Paris: 1st class on 06.25, 17.31, 18.31, 20.01

No standard class availability and if you have 2nd class pass buying/using/upgrading is not allowed.

Eurostar just ended a flash sale so a lot of seats were sold cheap in the last week, now there is poor availability with almost all trains unavailable to passholders.

Alternative travel options:

Fly,

Train+ferry+train via Dover - Calais or Harwich - Hoek Van Holland.

Bus; Flixbus or Bla Bla Car (yes, someone thought it was a good idea to name a bus company that)

The weekend of 17 and 18 december there are no trains to Amsterdam. A lot of engineering works happen these last weekends of the year, further limiting availability.

Shopping for the final holidays of the year are a thing. 

The option via Lille is also fully booked for pass holders. There's Availability about 4 days earlier or 2 days later. Guess you'll need to look into the alternatives mentioned above or buy a full fare eurostar ticket. 

It is for many the last day to need to WORK in this year-next week the normal Xmas hols start etc-so demand is extra high on such peak moments. Is that so hard to even think of?

Expect the very same on 1/1 amd 2/1 in the Nw Yr to come back to that funny island in the North Sea on an eco-friendly matter.

The worst option is the ferry via Calais. If travelling on a budget book a coach or try to get a cheap flight.