Hi,
As it must be sent by the browser, comes to my mind to set in the hosts file a line like:
127.0.0.1 hostname.somewhere.abc
And replace in stunnel service configuration the connect = hostname.somewhere.abc:443 for connect = IP of hostname.somewhere.abc:443 (hostname.somewhere.abc now points to 127.0.0.1)
Could work unless dynamic IP on hostname.somewhere.abc or shared hosting, that would be a headache.
Or try other ways to spoof Host http header sent by the browser. A proxy is not an option as it preserves it.
Regards.