Ok, I just tested this with a simple arm with a custom attribute in the IK-handle that effects the hands rotation, so it should work in your situation as well.
- Make a temporary cube. Be sure it is positioned in the middle of you character.
- Make the original arm, and it's IK-handle(s) a child of the cube.
- In Duplicate options, check Duplicate Upstream Graph.
- Duplicate. This will dupe the cube, and it all it's children (the joints, and IK-handles)
- With the newly duplicated cube still selected, set it's scaleX to -1 (that is if X is left and right for this character. Otherwise, it's probably Z).
- Unparent the new arm and it's IK-handles from the cube and make it a child of it's proper parent.
Theoretically, the new arm should be fully functional, with all the attributes, as the original.
I hope this answers your question.
I have no idea how to change the properties of an existing attribute although Axel's replay (6/26/00) to KUBBA's post "Renaming attributes" 6/25/00 may give you a clue.

paul