I have managed to share keyboard/mouse between 2 machines, of which only one of them is connected to VPN (OpenVPN client). Both machines are on the same subnet, running Linux, and connected to a Linux firewall running shorewall.
Add the following to the /etc/openvpn/client.conf on the client:
Add to /etc/shorewall/interfaces on the firewall to allow LAN machines to talk to each other.
Finally start synergys on the server, and synergyc 192.168.2.11 on the client.
For additional security one can use a ssh tunnel to encrypt traffic. On the client: