Question

Have a good week,
in the army it is common to use a "Sabbath pen"; I assume you know it. The ink disappears.
I wanted to explore the possibility of using it at home. May I ask for your view on the issue from several angles?
First of all, in your opinion is it forbidden at the Torah level or only rabbinically? Could you spell out the sources for the prohibition either way?
 
If it is rabbinic—what could nevertheless justify it? For example, could a very clear financial need, or an acute emotional need, justify it, even if it is not quite a matter of saving a life?
If at all, of course.