Hi David,
If you want to pass through the SSL unencrypted when pool members are available, but terminate the SSL and send an HTTP response if the pool is down, you could import the SSL cert and key, configure them in a client SSL profile, add the client SSL profile and an HTTP profile to the VIP and then use an iRule like this:
http://devcentral.f5.com/wiki/default.aspx/iRules/HTTPS_passthrough_fallback_URL.html
Without importing the cert and key, there is no way to inspect or modify the HTTP headers or content.
Aaron