I'm currently way over my head in terms of work, but if someone would like to do this I'm just putting the idea out there.
I hate having invincible companions almost as much as I hate them dying in under 3 seconds into a fight.
So, what if we found a middle ground? Have a companion have a large amount of health. But, when his or her health reaches a certain threshold, they are knocked unconscious and will bleed out for about 30 seconds to a minute (perhaps reliant on player charisma and companion endurance). If the player manages to 'activate' the companion in time with a stimpack, he or she will fight another day. Otherwise....
This wouldn't be hard to do, just a matter of making it applicable to all companions
Thanks!
WCStorm11