I'm not sure the French would tolerate a USB. Part of the (French) revolution's ideology was that Belgium was rightfully French, that it had been 'stolen' by Louis XV's status quo ante bellum treaty after the eponymously 'stupid' Peace of Aix-la-Chapelle. Republican or no, fellow travellers or no, I don't think it would make that much difference.IIRC the Austrian Netherlands had a revolution in 1790 that resulted in an independent United States of Belgium for almost a year...this revolution could conceivably be delayed to a point when Austria was too weak to put it down, providing another republic in Europe?
Perhaps Poland isn't partitioned away?