I think this is a feature limitation, and my suspicion is that it requires that the Drop Cap be aligned at the starting edge of the text frame (so that you can drop on the right end of RTL text). How would you propose to deal with a case where the text is right-aligned, but the first line is significantly shorter than the second?
The fast solution I will adopt is just to put a capital only for the first line.
How would you propose to deal with a case where the text is right-aligned, but the first line is significantly shorter than the second?
think is not a big deal as these text are captions for pictures and the space is variable.
It might not matter in your particular case, but it is a question that needs to be considered to implement a left-end drop cap on right-aligned text.
I think you might be able to fake it, too, by using an inline frame holing the drop cap and applying some text wrap to it.