You can use caravans to ferry items from a city to the other. I know, that doesnt adress the issue, but I found some limited uses for it...

out of curiosity, other than its use as a quest item, what is a midnight stone good for ?

joy, i thought it might be useful in a fight - guess not. too bad you can't buy them, i routinely have them as an open ended quest with no graveyard sites to find them - i have yet to see the sword or shield you get at the end of it.... 

I have done this quest a few times.

In a single game you can do this quest multiple times, my max was 3. It seems to be a progression type of thing. Each time I got the sword first, then the shield then a plate curias. I am not sure what happes if you continute to do more of them.