In principle you can do proxy ARP. linux 2.4 will automatically do proxy arp when you set an interface route for the respective network and if ipv4 forwarding is enabled.
so you can just put a route for your ISP's network into your LAN interface, and ASL will forward the packets from the opposite interface into your LAN.
However, this is a "dirty" solution. Better ask your ISP for a small transfer network. Even a transfer network with private IPs (192.168.x.x) will work (but it's not so nice to bind services to it [:)].
