Hi and welcome to he Pukkelpop thread.
Late and night there are no trains, so I think that the best idea is to try to find a room for friday night in Hasselt and then move on saturday morning to Brussels.
There is a free shuttle bus from the festival to Hasselt that works perfectly at night. Besides, your train journey will be free with the Pukkelpop ticket (you will receive in early summer a link from you can download your train tickets).
Maybe will be cheaper a room in Hasselt than the taxi trip + the hotel night in Brussels.
Feel free to ask everything you need. Here there are really nice people as @Semprini, @DDave, @Trufflehound... that have attended the festival lot of times.