How to change the "?" representation for a missing cell?

Hi forum,

how can I change the representation of missing cells in an output table?  By default, they are shown as "?"

I did not find something like a MissingValueRenderer.  The question mark string comes from MissingCell.toString(), correct?

I thought of maybe deriving MissingCell in order to override toString(), would this be feasible?

Thanks for all tips, Frank