No, sorry. Currently it's all hand-coding.
It would certainly be cool to have an editor, but I'm very short in time currently, so I better not make any promises ;)
so basically i need to man up and write one ;)
I already have a 1/2 implemented XML loader. (specific attributes, and only on some controls).
I suppose it couldn't too hard to add a wrapper around this for exporting to XML and importing and creating the dialogs.
If you have any interest let me know. I can be found on irc.freenode.net ##xna.