The CSS automatically detects failure of a next-hop gateway. Detection of interface-down on a next-hop router is not a feature of the CSS, but it could be addressed in two ways:
1. CSS could learn a dynamic default gateway from the 7200, which is a static route on the 7200 (via the serial link) redistributed into an IGP.
2. CSS functionality can be extended with scripts, using the advanced features. The "WebNS Advanced configuration Guide" contains a section on CLI scripting. The scripting language allows the CSS to modify the running configuration based on events it can detect. So a script could be written that attempts to ping the serial interface, or the far-end of the serial link, and then enters (conf t) mode to add or remove a static route to 0.0.0.0/0 depending on success/failure of the ping. Custom scripts can be used to augment the functionality of the CSS, but only the canned scripts that ship with the system are supported by Ciscos TAC. Use the canned scripts to learn the syntax if you wish to try your hand at scripting.