Okay... now I understand! The answer would be no. I say this loosely, but, when using WebAccess, you are not using a "GroupWise Client", you are using a browser. So you dont have the same setting as you would normally with the actual GroupWise Client.
Also, I'm not sure if you can do what you are asking in GroupWise Remote.
The Proxy function is strictly a GroupWise Client Application setting. You will notice that you cannot even setup GroupWise Proxy settings through NWADMIN. Only the Client itself.
Good Luck!