Even if the tool does not check the version number in the request, wouldn't any proxied response contain a HTTP/1.0 or HTTP/1.1 header? In that case, the client will see all but the first request in a connection stands no chance of being fulfilled, and fall back to HTTP/1.1 behaviour (perhaps caching a NO_HTTP_1_2 flag per hostname for a while)

Edit: Or also add an "Upgrade: slices" to the initial request, and if the expected response to the Upgrade: request is missing, continue with HTTP 1.1.