The reliable Seat61 website has announced that from 12th October (I assume bookings made on that date rather than travel on that date) Eurostar tickets between London or elsewhere in the UK and Brussels will no longer give a free same-day connection to/from any other Belgian station. Apparently from then you will have to specifically book connections and they will have a standard additional fare of £5.50 each way standard class or £7.50 each way first class. No comment on whether this affects the similar offer for Thalys trains linking Brussels with Paris, Amsterdam and Cologne.
"Apparently from then you will have to specifically book connections and they will have a surcharge of £5.50 each way standard class or £7.50 each way first class." On Belgian rail, this should not even be necessary. Trains leave in every direction from the Brussels rail stations every few minutes, so no matter where you want to go, you rarely have to wait longer than 30 minutes. Just go to the ticket counter and buy your connecting ticket when you arrive. Voila, no surcharge.
Sorry if I was misleading, it looks as if that isn't a surcharge on top of the Belgian rail fare, but that that is the Belgian rail fare. I think it will be cheaper than buying a ticket at Brussels unless you're going a very short distance. I've edited the original post to correct the confusing bit.