In ESXi 5.5 and 5.0 it is in /etc/vmware/hostd/proxy.xml. In 5.1, it is nowhere to be found, you are correct. I've uploaded the 5.0 version. Perhaps you can try to upload that to the host (in the /etc/vmware/hostd folder) and see if that works.Not sure if this persists on a reboot though:
Proxy.xml can be found here: http://tomverhaeg.nl/files/proxy.xml.txt