As long as you keep the default setting, it ought to just work. Do you see problems with this? The setting in question is
CONFIG proxy.config.http.send_http11_requests INT 1
To make sure I'm not smoking crack (this time at least...), I tried it (sending an HTTP/1.0 request to ATS). Below is a debug tracer from traffic_server, as you can see, the client request is HTTP/1.0, the proxy request to origin is HTTP/1.1, the response from Origin is HTTP/1.1, and finally, the response back to client is HTTP/1.0 (which is a must). This is why conforming to RFC2616 should not be considered optional ;).