Does owncloud sync clients support SSL Client certificates?
there is currently no support for SSL client certificates in the desktop sync client:
You could look at using a VPN (there are many desktop clients based on openvpn) to connect to the server, and redirect the traffic through the vpn tunnel. That would give you an equivalent level of authentication as each client computer would have its own certificate, and vpn software is well designed and tested against attack.
You could also use the same method for https traffic, and not expose the own cloud to your local network directly at all.