So I probably don't have the exact answer you are looking for as honestly I don't have too long to go digging around with Workgroup Manager, BUT, I may have some food for thought for you. Have you considered using the 'networksetup' command? Launch Terminal and type 'man networksetup' and you'll see it's syntax. It offers rich abilities to set proxies as a one time deal on the client. If you want to go the MCX or profiles route, that is fine too, but I don't seem to see proxies for Ethernet as a template or as a profile option.

We use networksetup in a shell script. Works great.

We tried Configuration Profiles, but as Brian mentioned, nothing for Ethernet. Even with Wi-Fi, we found its actually modifying the setting to be sporadic.

We are using a script to push out these settings. This is what we have setup. We use AutoProxyDiscovery. This is for Wi-Fi and then we have one for the ethernet. Works great.

#!/bin/bash

networksetup -setproxyautodiscovery Wi-Fi on

We use http://macmule.com/2011/09/09/how-to-change-the-automatic-proxy-configuration-url-in-system-preferences-via-a-script/

Thanks guys. I had also found a script using networksetup. I Think JAMF should consider this option in Configuration Profiles.

the link http://macmule.com/2011/09/09/how-to-change-the-automatic-proxy-configuration-url-in-system-preferences-via-a-script/

is not accessible anymore

the link http://macmule.com/2011/09/09/how-to-change-the-automatic-proxy-configuration-url-in-system-preferences-via-a-script/

is not accessible anymore

Here is the working link:
https://macmule.com/2014/12/07/how-to-change-the-automatic-proxy-configuration-url-in-system-preferences-via-a-script/