Bone snapping tool. Works for parented bone, only if there's no Child Of constraint involved, and all parent's scale are uniform across all axis.

gistfile1.py

import bpy

class VIEW3D_OT_BoneSnapToObject(bpy.types.Operator):
    """Snap active bone to selected object."""
    bl_idname = 'object.snap_to_object'
    bl_label = 'Snap Bone to Object'
    bl_options = {'REGISTER', 'UNDO'}

    @classmethod
    def poll(self, context):
        return len(context.selected_objects) == 2\
            and context.mode == 'POSE'

    def execute(self, context):
        target = [o for o in context.selected_objects
                  if o != context.active_object][0]
        active = context.active_object

        mat = active.matrix_world.inverted() * target.matrix_world

        pbone = context.active_pose_bone
        for p in pbone.parent_recursive:
            mat = p.matrix_basis * mat
        mat[3][0] = mat[3][1] = mat[3][2] = 0
        pbone.matrix = mat

        return {'FINISHED'}

def register():
    bpy.utils.register_class(VIEW3D_OT_BoneSnapToObject)

def unregister():
    bpy.utils.unregister_class(VIEW3D_OT_BoneSnapToObject)
    
if __name__ == "__main__":
    register()