Hello,
I don't think this is possible with the out-of-the-box Alfresco, but it should be a comparatively simple exercise to implement a custom Share SSO filter, which checks for the users "origin network" before prompting for SSO credentials. This would require a custom implementation of the (Java) filter SSOAuthenticationFilter and potentially a custom HTTP header provided by your SSL gateway proxy server to check for.
Regards
Axel