Take a look at the question mark on the GoDaddy link next to the Multi-domain option. The pop up that appears seems to have a limitation but isn't really detailed or specific. You could look at
http://www.geocerts.com/ssl/tbidmd and the FAQ section on the right side for another provider's details.
The details of the domains, their owners, and how you want to use it are key. As I understood this you would basically be like a web host provider with completely different, unrelated domains hosted on a single server. If so it doesn't make sense you would find a certificate that would "sign" all of those for one small price. Each is a different company and basically needs its own. My suggestion of a 3rd party would be a way around it if acceptable to the clients and their visitors. You might also be able to offer discounted prices if you would be selling a number but they usually each need to register. Of course this does all assume the domain owners want their info to show in the certificate info. I definitely am not trying to say there isn't a product like what you asked about. The expert was right to mention it. Its use just seems to be different from what I believe you are thinking or intending to do.
I hope this helps but you can always clarify with the certificate authority to get details or clarification.
bol
