Yes, you can mantain user list that made an RPC API call. If you go through with the ZoneAdapter, you will find you can fetch zone users by calling API iZone.getUsers().
In Zone RPC call you can send userName as a parameter. In RPC method you can fetch that iUser by itereating iZone.getUsers() collection. Once you will get that user after that you can make your own iUser collection based on RPC call.
Let me know if it helps.
Thanks & Regards