Shouldn't be too hard to do. The only hard part is the auto-update, you would have to use a reactor. If you don't want to go that route, then maybe you can have the user either select an existing text to overwrite, or place it on the drawing. You won't be able to use this for LT (as you can't use lisp). I don't know diesel, so I'm not sure about that.
Psuedo code.
Use entsel to select the xref.
Use tblobjname with block and the name of the xref
Use findfile once you get the path listed for the xref
Use vl-file-systime to check the date of the xref drawing
Prompt the user to select existing text, or place new text on the drawing.
You may want to create a new layer for the xref text, and then when the program is ran, it can check for any xref (ssget), and then once it gets those, it will search your drawing for that one layer, and erase all the text, or rewrite the text.
Hope that gets you started in the right direction.