Cisco Firewall :: How To Filter L2L Traffic To A PIX 7.2(4) (or ASA)

I've got a PIX running 7.2(4) with its outside interface on the Internet.  The only thing this PIX is doing is acting as the endpoint for an IPSEC LAN-to-LAN tunnel with an Internet-connected ASA on another network.
 
I'd like to filter inbound Internet traffic to this PIX so that only the designated ASA can attempt to establish an IPSEC connection -- in other words, I want to prevent any other device on the Internet from even being able to attempt to establish an IPSEC connection to the PIX.  As far as I know (and have seen), this can't be done with an access-list on the outside interface, since that access-list doesn't apply to traffic to the PIX itself.