In the end I bought a Dutch ticket to Roosendaal (NL), and at the same time I bought another ticket from Roosendaal to Rotterdam for the next day.
There is an SNCB machine at Roosendaal, but unlike SNCB machines in Belgium it only takes card and not coins. You can also buy forward dated tickets.
My journey was cheaper by about 10 euros to split at Roosendaal compared with buying a through ticket from the both Belgian and Dutch machines, and if you have all your tickets in advance you don't need to leave the station, as you can check in with a OV-chipkaart (including disposable singles) on the platform. If you are taking a through train and not the SNCB stopping service between Roosendaal and Antwerp/Puurs, there is plenty of time to tap on the pole and reboard the train you arrived on. If you know what you are doing you could even exit the station, buy your next ticket and get back on the same train.