Stephan,
We do already have an option here: https://github.com/maths/moodle-qtype_stack/blob/master/stack/options.class.php#L285
which is implemented here: https://github.com/maths/moodle-qtype_stack/blob/master/stack/maxima/stackmaxima.mac#L233
So, if you have chosen that option at a question level then it should reflect validation display. If you want another (!) choice through this option it should be straightforward to add it in parallel to the existing choices.
Chris