My idea was to increment the value via select message. Decrement would not be possible. So your xmenu_value callback should also handle the select message and increment the value accordingly: This means you can navigate to the menue entry which has the xmenu_value callback attached and then press select to cycle through the values. Of course this makes only sense if the number of values is limited.
The initial idea of m2tk was not to have a scrollable list of editable values. Instead the idea was to have several independent pages with data entry fields.
Oliver