this is not a problem a missing feature.
This is a general issue.
With SSL traffic, if you do not have an SSL module, the only information available to the CSS [or any other loadbalancer from any vendor] is the information contained in the IP,TCP and SSL headers.
There is nothing useful in the TCP header.
The only option in the IP header is the source IP.
And for the SSL header, you could use the SSLID.
However, this last option is very reliable as Internet Explorer is well-known for changing this ID very frequently during the same session.
Therefore, the only possible choice is source ip unless you can get an SSL module, then you can decrypt the traffic and use cookies.
Regards,
Gilles.