Thing is, what else would Austria and Russia be going to get, even if they take Paris. None of them would want Alsace, I guess, unless France resists more than an year or so. If Paris is taken, say, in the fall of 1816 by an Austro Russian army you can have a Carthaginian peace for France and Britain having little say in such things as the fate of Belgium. Still, the Wacht am Rhein would be Prussia's task. Austria and Russia will have compensation further east, but there will be problems: Austria is unwilling to let the Russians expand further into Poland and is eager to keep and indepent Saxony as a buffer between herself and Prussia. Russia on the contrary would like to annex all the former Duchy of Warsaw, offering something else to compensate Prussia for it. Alsace-Lorraine might be fine, Belgium would be a possibility.
In the event of a punitive peace, French losses in the colonies would still go to British advantage in most cases, with the exception of Guadeloupe (primised to the Swedes). The Netherlands also might get a chunk.
Austria does not want Belgium back in any case.