Right now our only options are to either capture an allied faction's colonist, or watch him bleed out on our lawn. How about the ability to rescue these envoys? We take them to a medical bed instead of a prisoner bed, and once healed they return home. Upon leaving the map area rescued colonists give a bonus to relations with that faction. Alternatively you could try recruiting them as they heal, but since they're free to leave you only have a limited time convince them to join you.
I think the dev knows about this feature as it is on everyones mind, people hate watching freindlys bleed out when they have meds and possibly something to gain.
It been requested many times don't worry it's probably going in.