Is it possible to provide network data encryption for only one of 3 databases on a server?

We are running 3 databases on an AIX server for 3 separate customers.  One of the customers has asked us implement network data encryption between their clients and the database server.  They do not require encryption in the database. Our other customers on this server do not require network data encryption, and we prefer to install and configure Advanced Security on a minimum number of client workstations.  The database level is 10.2.0.4

We first thought of running multiple listeners as a way to accommodate this request, but it seems that the parameters to configure Oracle Advanced Security encryption and integrity on the client and server are in sqlnet.ora vs. listener.ora.  So all listeners running on the server out of a particular Oracle Home would be under the jurisdiction of OAS if we installed it and configured it as being "required" on the server.  It doesn't seem we'll be able to isolate Advanced Security to a listener on a different port.  Are we correct in this thinking??