Does the "Any Belgian Station" destination on the Eurostar imply unlimited Belgian train travel (naturally excluding Intercity-Express and Thalys) within 24 hours of arrival/departure of my Eurostar at/from Brussels?
This ticket gives you 24 hours, after the arrival in Brussels, to reach your final destination in Belgium. You cannot use it for roaming across the country.
A word to the wise: I am not a professional translator, but it seems to me that there may be an ambiguity between the English, Dutch and French versions of the "conditions of carriage". In the Dutch and English versions they mention travel within Belgium. In the French version they say something like "one trip within Belgium". If you are really stubborn you can use this as an argument, but I doubt that you will succeed ...