I think it is very wonderful your group wants to save her, I really do. For her to be in that kind of pain would be horrific, she seems gentle enough to allow you to handle her to treat it.
That said, if this horse came through our rescue I would not hesitate to demand she be put to sleep. We believe horses should die with their dignity. This is not dignity, this is suffering. I do not believe that this wound will heal without major issues down the road. The animal has no hoof wall left, and there is exposed bone. Without access to a top notch equine hospital and state of the art medicine and surgeries, I cannot see her surviving.
If I had no choice but to try, I would consider having the leg amputated midway up the cannon bone and having her fitted with a prosthetic limb. I cannot see that injuring healing enough to allow her to walk on it.
For sure- get her out of that sand. Those fine granules are going to work into the wound and carry bacteria. I would bed her down with shredded paper myself.
Good luck. I'm sending as many blessings as I can come up with for her.
Spent a whole hour today laying in a pasture, waiting for a sparkling vampire to show up. Alas, I woke up and looked over, only to find a mound of horse crap. Sigh.