Apache HTTPS reverse proxy with SNI without key on the proxy
Do you really need Apache reverse proxy, or you need the problem solved? I had the same problem and I resolved it with HAProxy in tcp mode as described here http://blog.haproxy.com/2012/04/13/enhanced-ssl-load-balancing-with-server-name-indication-sni-tls-extension/ instead of Apache reverse proxy.
If you don't mind using Nginx or HAProxy instead of Apache, you'll find good answers at the following question at ServerFault: