Since neither the feature request nor the patch list contains a relevant entry, I assume that this feature is not on the todo list.
In principal it should be easy to add the missing methods to the CrosshairState class, but the additional series and item index are meaningless for a CategoryPlots. The CategoryPlot uses an instance of CategoryCrosshairState for tracking which offers to store the row key and column key.
Consequently, one could use an instance of XYCrosshairState for an XYPlot. Interestingly, this class is already included in JFreeChart. It has no methods except a non argument constructor, but the missing methods could be easily added. Unfortunately, the drawItem method of XYItemRenderer only uses an instance of CrosshairState, not XYCrosshairState. Changing that would break the XYItemRenderer interface!
Since the CrosshairState is only used in AbstractXYItemRenderer.updateCrosshairValues(CrosshairState, double, double, int, int, double, double, PlotOrientation) one could still perform the necessary cast from CrosshairState to XYCrosshairState in this method.
Once the series and item index are stored in the XYCrosshairState instance, it should be trivial to add the missing private fields and public getters get[Dataset/Series/Item]IndexAtCrosshair.