If you don't want it to be triggered by others, but during combat, RCs rules will only provide a starting point.
But I think the simplest solution is to say that a character can, once per 5 minutes (or encounter, if you can live with that "metagame" term), get temporary hit points equal to half his hit points, but only if he has less then half his hit points (and isn't dead yet, unless you want someone to stand up after a finger of death

). These temporary hit points do not stack (same source rule). They disappear after 5 minutes*.
Note though that if the discussion has been going into a direction you didn't like, it was because we talked on "different levels" - from a general game design concept to a specific discussion on hit point mechanics to you actually hoping for clear suggestions on how to implement a second-wind mechanic in your game. Nobody can hope for sensible answers if the other side is actually not aware of his questions.
*) What did Robert DeNiros character in Ronin say after he helped Jean Reno stiching him up? "If you don't mind, I'll pass out now." (?)