Do I get this right, you are talking about players meeting in solo and spontaneously agreeing on NOT shooting each other but teaming up til the end?
I must say, I find this an absolute legal, lawful and clever approach. And appealing to try, too, having a non-gurantee temporary brothers-in-arms deal, with a serious friendly fire trait working.
I understand, you want to secure fair chances in that matter. I, on the other hand, dig that chaotic element. It's difficult enough to accomplish, I guess (never tried). You need to find someone going along. Imagine how many tries you need on that. When you succeed, it's earned, in my opinion, no ping system, no healing each other and stuff.