Upon further investigation, the problem may lie not in INJecting the ShaderNodeParms but in linking the material values to them. If I apply a version of the pose that doesn't apply the materials automatically but does everything else, then it doesn't crash when I try to save it. But when I then apply the material collection separately, the links between the shader values and the parameter dials aren't there

I suppose I shouldn't be surprised. I've sometimes experienced those links randomly breaking after saving a PZ3 or CR2 anyway, even before I began experimenting with creating those links via INJection pose.