Hi Peter,
Seems like you are trying to send mail outbound but want to change the recipient address using address masquerading, right?
As I quoted earlier from the Admin Guide, the address masquerading changes the "sender address" in an outbound message.
If you want address masquerading to change the "recipient address" you will have to send the mail inbound, which means you will have to configure the fake domain/email address as the local address and then use Optional Destiantion Routing to deliver the message to proper destination.
Hope this helps.
Thanks
Adnan