Perhaps the easiest way to get rid of Dutch Belgium is to simply destroy the Netherlands during the Napoleonic Wars. If the would-be King William I follows his father's lead and supports collaboration, instead of being a figurehead for Dutch resistance, perhaps the Netherlands itself could go the way of Venice or Poland.
Such a scenario does have the difficulty that Austria had ruled Belgium before the French Revolution - indeed, it'd make far more sense to give Prussia the Netherlands and Austria Belgium, given that Prussia had marriage ties with the last Dutch hereditary stadtholder and had saved the last stadtholder's power in 1787. But it was non-contiguous and not all that inclined to Austrian rule, so a trade isn't outside the realm of possibility.
Except that it leaves the question of what to do with the former Dutch Republic. No one wants Prussia annexing it in addition to Belgium. Its other major neighbor is Hanover, but Britain doesn't want the Netherlands proper, just its colonies (which it already took).
Prussia has neither a historical claim to Belgium nor much of a strategic rationale to emphasize taking it over, and although it borders the Prussian Rhineland, that region itself is poorly connected to Prussia and it makes more sense to scoop up the states in between. And that's a big hurdle to overcome.
I think the best option for Prussian Belgium is to get it the same way Austria took Belgium - which is to say by a dynastic inheritance of the Burgundian state, of which Belgium is the bit left over after the Dutch rebelled and the French expanded. This presents its own challenge - namely, that there was no Prussian monarch of any sort at the time. But perhaps if the Teutonic Order can secularize without being reduced to the tiny Duchy of Prussia of OTL, and the Burgundian royal line sputters on another century or so, a larger Prussia could marry into and with luck inherit Burgundy.