I agree with you that 1848 is a really good place to start. Archduchess Sophie was already somewhat of a political finagler, getting her own husband (who was reportedly feeble-minded) to step down in favor of their eldest son). We just need her to get the revolutionaries to accept Franz Josef, or preferably Maximilian, who was by far much more liberal than his elder brother, to accept the imperial crown. Prussia would probably immediately declare war, and possibly some kind of alliance between Grossdeutchland and Poland can be formed under the grounds of a division of Prussia.