It seems that there are two requirements mixed in a single request. One is for creating an editor which can help edit XML and the other one is integrating it into the grid. If the XML is interpreted as a string the second part is already supported as one can use a custom editor with the grid.
As for creating such an editor for XML I would kindly like to ask you to provide more details on the functionality that it requires. If you have any samples or ideas that you can share with us that would be great.