POD: The Prince of Orange dies at Waterloo. He was wounded IOTL.
Things roughly follow OTL, until 1830. An urprising breaks out in Brussels in July/August. It quickly spreads throughout the southern Netherlands (Belgium). King William I, not being hampered by his flip-flopping son (who died at Waterloo) doesn't back down. Maybe the death of his heir hardened him or something. Antwerp, Maastricht and Venlo are bombarded by the Dutch. The Belgian provisional government is drawn up earlier. This means some delegates don't make it to the congress as fast as they did IOTL, such as Louis Joseph Antoine de Potter or Sylvain Van de Weyer, both of whom were out of the country at the time. So, someone other than Charles Rogier is likely elected the provisional prime minister. Let's say they elect Alexandre Gendebien, a francophile. This causes the de Mérode to be made regent of the country instead de Chokier. With the Dutch still besieging the country and the fledgeling Belgian forces unable to throw them off, the provisional government is more desperate then OTL. They call for foreign assistance.
IOTL the Belgians offered the throne to Prince Louis, Duke of Nemours, the son of the Orléanist King of France, Louis Philippe I. However the French turned it down. With the Belgians still fighting the Dutch ITTL, they have even more reason to do so. The National Congress' two other top choices IOTL were Auguste de Beauharnais, the then Duke of Leuchtenberg, and the step-grandson of Napoléon; and Archduke Charles, Duke of Teschen, the last Hapsburg Statthalter of the southern provinces. Beuharnais was a close second to Louis IOTL, so he's likely to get the nod ITTL.
Beuharnais leaps at the offer, despite British protests. Now he lived/ruled already in the Bavarian 'principality' of Eichstätt, so he'd probably arrive in Belgiun sometime in early spring 1831 (BTW, with the fighting on-going between Belgians and Dutch, the National Congress is likely held in a more southern city - say, Charleroi. Actually being in person at the congres, Beuharnais would be able to swing votes his way to getting elected to the throne. Now with their own King, likely elected sometime in April or May (2-1 month ahead of OTL), and their own constitution, the Belgians would begin to push back.
At this point the Dutch begin to panic. There's been no London Conference ITTL, and the Dutch have never truly let up on their attempts to retake the southern provinces, so there's nothing akin to the Ten Day's Campaign ITTL; at this point Dutch forces on the continent are largely spent or held down attempting to take the major northern Belgian cities such as Brussels, Ghent, and Antwerp (the latter of which has likely fallen), or have deserted (IOTL some 2/3rds of troops from the southern provinces deserted - that number is likely even higher ITTL. We're looking at a desertion rate of perhaps 80%, which means effectively one-half of the Dutch army, including their arms and material, is now fighting for the Belgians). The colonial forces are too far away to be of any use at this point, if William has even summoned them yet. So the only option left to the Dutch is foreign assistance.
Even at this point IOTL William I knew he wouldn't be able to retake the southern provinces, and merely wanted to negotiate from a position of strength - that's likely to be the same ITTL, so he's going to call upon aid from someone who isn't going to be able to, or won't, hold it over his head at a later point (or at least not to a degree that he would find unacceptable). So that means the British. Palmerston, being Palmerston, is going to say no. He doesn't want to piss off the French, and he wants to maintain the balance of power on the continent. The idea of the Brits getting martially entangled in some affair across the English Channel is not something he is going to be interested it.
So now the Dutch have no one left to turn to. The Prussians are too busy, and rather too small at this point. Still paying off their war debt from the Napoleonic Wars, involved with the various German states in trying to set up the Zollverein, busy guarding her eastern border and watching over her Polish subjects due to the outbreak of the November Uprising.
So the war carries on for a time longer than IOTL, but ultimately no great intervention comes of it. Hell, the French likely never even get involved; Étienne Gérard expedition IOTL was a direct response to the Ten Day's Campaign. Ultimately the Dutch aren't going to be able to reconquer the Belgians, but the Belgians won't be able to reclaim all of their lost territory. A general cease-fire is likely introduced sometime in late 1831/spring 1832, with the new borders established where the front lines where. The Dutch keep all of Limburg and Antwerp, and some northernl parts of East & West Flanders, the Flemish Brabant, and Liège. The Belgians get their OTL country plus all of Luxembourg. The capital of this new country is...Mons? It doesn't have a coastline, though...
The Great Powers accept this because it keeps the peace and contains the revolution. The French aren't going to attempt to integrate a 'Napoleonic' kingdom into their newly established Orléanist one, Talleyrand be-damned; the British are simply interested in their own affairs and making sure the balance of power isn't over-turned by the 1831 Revolutions; the Russians (and Austrians) can do nothing to stop the outcome; and the other European states were hardly 'great powers' in 1830-1832. King William, disgusted and physically and mentally exhausted, resigns from the Dutch throne in 1833.
