As of today, with the current version of HCL Sametime Proxy Server, there is no supported Sametime configuration which will enable you to send APNs or GCM push notifications via Web Proxy server. This feature will be implemented in future releases.
Keep that in mind when planing your Sametime Deployment.
HCL Traveler and HCL Connections support this functionality, as documented in official documentation:
HCL Traveler – Push messaging through a proxy
HCL Connections – Mobile configuration properties for HCL Connections 5.5
HCL Connections – Configure access for the Tiny Editors Services through HTTP proxy