Make a rule with whatever criteria you want it to match on, destination, site, client.ip, etc. (Request Cycle)
Create an event that sets the property Request.ProtocolAndVersion:
Set Request.ProtocolAndVersion = "HTTP/1.0"
Must be something else going on. That alone doesn't make the site work. I have a workaround for the customer so will revisit.
You also might want to try the proxy control event and check the options in there.
Specifically, one of the 1st 3 options:
override chunk encoding
persistent connection handling.
Seems that there was something related to browser identification. I didn't directly work on the final solution, but as I understand it, if the user-agent was IE, something (perhaps Accept-Encoding) would get munged, but only in WCCP mode. Implementing a user-agent rewrite at MWG served as a usable workaround.