I don't think you could make an xnode that behaved like a structure without a lot of trouble.
One nutty idea would be an xnode that when dropped creates a sequence frame in its containing diagram and use that for your basic structure functionality.
Then if you can draw the xnode on top of the frame entirely without it looking terrible, I suppose you could intercept mouse interactions with the frame. Or maybe there is a way to get notification of when the frame is resized, moved, deleted etc and adapt appropriately.
