No, it's not possible at the moment, only the display color of the objects is exported as user data.
I can add the index as well, if we agree on what the index is, since this concept is missing from the SDK. And it's not as simple as it seems. The Cloner generates an object tree, so the index can be based on a breadth-first traversal.
What about objects not exported to Arnold (e.g. Null objects), do they have an index or not?
What about Cloner under a Cloner, where does the index come from?
Also I'm not sure if the generation order is well-defined, so it might be that the order in the tree is different when the Cloner changes (e.g. different number of clones, objects are hidden, a different effector is used, etc.).