Using the uitextarea <callback function> would be the only way, but I don't think you will be able to make it work for the <\t> character; it is treated especially to change focus of the control as you are awared. On top of this, to do it with the ValueChangingFcn callback, be aware of the following Note:
"Avoid updating the Value property of the TextArea object from within its own ValueChangingFcn callback, as this might result in unexpected behavior. To update the text area value in response to user input, use a ValueChangedFcn callback instead."
It would seem you would have to not only trap the character but if it were a tab, then reset focus back and then where is the user cursor on entry, etc., etc., etc., ...
Hence, while I doubt if even this would work well, the only way I see one might be able to do it would be to keep a running position counter and then insert tabs in the object when the user exits...
My guess this is simply not going to be feasible; you would need a custom-written UI component methinkks.
