My family of four plans to travel from Brussels to Liege on 2 June 2013. We have eurail passes. Will we need seat reservations for the Thalys? If yes, how far in advance are the seats likely to fill up/sell out? Do we need the reservations a month in advance to get seats, or would a day or two in advance be good enough?
You will have to pay a large supplement to ride Thalys with a railpass. Make it easier on yourself. Take the regular intercity train between the two cities. No reservations needed (or offered). I doubt you would need to pay a supplement either. And you might want to re-think the railpass idea, if your goal is to save money.
As Tom says, take an IC train. They run much more frequently than Thalys trains and your pass will be good on any train at no additional charge.