FWIW, the limitations of https bindings to a single IP has nothing to do with SmithCart, DotNetNuke or IIS.
SSL negotiation (certificate exchange) occurs before HTTP and before the server knows which hostname the client wants.
There are no complete workarounds.An ssl certificate can be bound to a specific port. Wildcard certificates can use one certificate with multiple subdomains on one domain. SAN (Subject Alternative Name) certificates can secure multiple domains under one certificate. The last solution is SNI (Server Name Indication) but it is not widely available yet.
You can use a single certificate for multiple portals, but the name will be different than the domain.