I like the simple approach.

And as a practical matter, just chillin' is safe. But still:

  [sitting] can get uncomfortable. But when you finish, 
  for some strange reason you feel better

Meditation requires some guidance appropriate to the person. Some people are attracted to meditation for the wrong reasons; encouraging them without proper direction can make things go badly for them.

Zen shinkanzen in particular is considered difficult for new students because it is object-less meditation (unlike e.g. Tibetan use of mandala, Christian prayer, or Vispassana body-awareness). Even Zen teachers often start with breath-counting and progress to other heuristics before approaching emptiness.

People are generally robust to thoughts and opinions, even somewhat negative ones, so it seems like the marketplace of ideas and apps is a fine place for that. But I would encourage people writing meditation apps at a minimum to take the same care that any health/diagnostic app would, amplified by the likelihood that someone emotionally unstable might embrace your offering a little too closely...