Switching of only one tab might be problematic because, if I understand the process model correctly, the main process is the one that tells the others what to do, the renderer processes only render the stuff and execute JS etc.
... but maybe the following command line switches can help you in the meantime:
--proxy-auto-detect
Name says all
--no-proxy-server
Don't use a proxy server, always make direct connections. Overrides any other proxy server flags that are passed.
--proxy-bypass-list
Specifies a list of hosts for whom proxy settings are bypassed and use direct connections. Ignored if --proxy-auto-detect or --no-proxy-server are also specified. This is a comma-separated list of bypass rules.
--proxy-pac-url
Uses the pac script at the given URL
https://en.wikipedia.org/wiki/Proxy_auto-config
--proxy-server
Uses a specified proxy server, overrides system settings. This switch only affects HTTP and HTTPS requests