Archduke Karl and Grand Duchess Catherina as King & Queen?

[Marengo]

I am reading Adam Zamoyski's excellent 'Rites of Peace' at the moment and it is stated that Lord Aberdeen received intelligence that Tsar Alexander I of Russia and Emperor Franz I of Austria were planning a marriage of Franz' son, the Duke of Teschen to Grand Duchess Catherine Pavlovna, Alexanders sister 9later mother of our Queen Sophie). The couple would become King and Queen of The Netherlands...

I was wondering in what advanced state this plan was? As Alexander I actually offered part of The Netherlands to Denmark at one point I am not so surprised. Luckily there was England who wanted a strong Northern neigbour for France.