Thanks for this question. It's a challenging one because really RailClone needs to be able to generate or modify the geometry on the corner mullion, which is not really possible.
That said, here's a macro which uses a bit of a workaround to create something that I hope will help out. In order to use it you must create the corner geometry in 3 parts: a pointed start, a default section and a tail.
Just attach those and the other segments to the corresponding parts of the macro. You can see how this geometry is aligned and created in the attached file. Apart from the, using the macro should be pretty self-explanatory.
Hope it helps,