Well, if you want to really claim succession from the Commonwealth, any Szlachcic can be crowned. Maybe even hold a royal election.
More seriously, Polish monarchists were heavily divided as to who they supported, and they were none too popular even in the aggregate. That Pilsudski's circle was heavy with socialists, freemasons, and non-Catholics did not help (the Poles most likely to support monarchy detested the ruling government, despite the Pope's great personal admiration for the Marshal).
There was talk of putting a Romanian Prince or even Carol II on the Polish throne, and I think that, if you could seriously strengthen Romania and the Polish-Romanian Alliance, that might be your best bet--if Poland's constitutional government survives (avoid the assassination of Narutowicz?), the Sanationist dictatorship is avoided, and monarchy might come about legally and diplomatically.
Of course, the Romanian option puts the King on a collision course with the heavily anti-Orthodox Poles on all sides.