I do not believe that you can do that like you can with IOS. If the AAA servers are down, you should be able to use the user name of pix and use the enable password as the password to get it.
An alternative solution is to use local for serial console access only and use AAA for telnet and ssh (remote) connections, so that you still have access in the event that AAA is down.
See the pix password recovery doc, and be prepared in case you test AAA authen and you lock yourself out of getting access to the PIX.