Lookleft, I don't think changing flights to IFR is going to help, in fact it could make it worse by tempting pilots to continue when they should not.
Angel Flight is non-emergency, there is no pressure in these flights to get through. It would be preferable for standards similar to those suggested by Andrewr earlier, to be implemented. All of their pilots need to have it firmly fixed in their minds that they don't have to be heros; that it is far preferable to be on the ground sometimes; and that another 12 to 24 hours does not matter.