If you want to offer Shared SSL then get yourself another domain, anonymous domain name if you prefer and set it up with a certificate. Let your customers access the shared certificate with
http://anonymousdomain.com/~accountname/ .
Note that by allowing users to access their sites this way any bandwidth used will be put on the domain being used to access the site ie. the anonymousdomain.com.
If you have mod_userdir Tweak enabled to stop this you can exclued the account used for the shared SSL to enable it to be used in this fashion.