Though healing-minded drone mechanics may want to avoid taking specialization in longarms so they can use the needler rifle filled with healing serum (Mk. II or higher) darts, so they don't do more damage than they heal.
Though they'd be much better suited reconsidering whether they even want to do such a weird and kludgey build in the first place.
Slap a merciful on your needler, natch.
Not actually that helpful, unfortunately. Nonlethal damage is the same as regular damage, except that it can't kill you. There would be no different effect on a Merciful Needler than a regular one.
A regular needle shot would do lethal damage, take them to zero and giving them the Unconscious/Dying condition, and then the healing serum would kick in, removing the Unconscious/Dying condition.
A Merciful needle shot would do nonlethal damage, take them to zero and giving them the Unconscious/Stable condition, and then the healing serum would kick in, removing the Unconscious/Stable condition.
Functionally these are identical.