I have a similar set up:
client (443) ---CSS -- SSL accelerator -CSS - Server (6122)
I have http keepalives for the regular servers, and then I add a redirect service as the primary sorry server. Thus, when the normal server is down the user gets a redirect to a different web server that gives them a "sorry the application is down" message.
This works just fine with the Sonicwall SSL accelerators (which are proxies), but I don't know if it will work with the SSL modules in the CSS.
Best of luck.