The only effective sollution I can think of for this kind of security restrictions is to create and implement a fine-grained access control policy on that table. Look for terms "Row Level Security", "Fine Grained Acess Controll" or "Virtual Private database" in Oracle documentation and other available resources for more detailes how this feature can be used exactly for the purposes like yours.

The only problem is that RLS can't be applied to any table/view belonging to SYS. So in your case you would first need to move SYS.AUD$ table and its indexes to some other schema (like SYSTEM or something like this) and then create a synonym SYS.AUD$ pointing to this new table (to SYSTEM.AUD$ for example). Verify with Oracle Support if this kind of action on data dictionary would in any way compromise your support contract with them! But I know from the past that they allowed for this kind of AUD$ table move and that AUD$ was the only exception to the rule of "not messing up with database dictionary tables".