It looks like server name for client side SNI is defined as name to connect to in connect option. According to RFC, this name must be FQDN. It is the only place where it is defined and server name for SNI cannot be defined outside connect option. Is that right?
Server side SNI has some good examples. Searching for "client side SNI" did not yield anything useful.
I am trying to run multiple independent services over the same port there is no DNS infrastructure in place, so those server names would be random strings not refering to anything. Both ends would be stunnel. If it is as it seems, it will not work, because defined server name (at client) must be working DNS name (FQDN).