Just for the records, I found some more patches at JFreeChart's sourceforge-page that deal more or less with the same issue:
Which showed me at least two issue with my patch:
1) the equals(), clone(), read/writeObjecT() methods need to be modified.
2) the notifyListeners() method has to be called on changes to the plot which is not done within the current version of the patch.
An updated version (jfree-polarplot-v2.patch) has been uploaded to sourceforge.