This addon allows to copy the custom properties from the active object to the selected objects.
It also can take ob.data (all data types), and ob.data.bones, ob.data.editbones and ob.pose properties into account (on armatures). There is a separate button under armature which allows to copy properties from bone to bone.
Simply install as usually (Install from File) and you should find a new Copy Custom Properties Panel and Button under "Object" Properties and "Bone" Properties.
Its only a simple addon, but i hope it might be useful.
There is also a small Blenderartist Thread regarding to this addon....
Will add a wiki page later, but these are 2 new buttons only basically...