Here's some code I wrote in my mail editor to demonstrate how you can use ping-pong messages to only respond to an HTTP request after receiving confirmation that your imp got the message. It also uses a watched function to return timeouts if a specific timeout is exceeded.

agent.nut

responseQueue <- [];
timeout <- 5;  // timeout (in seconds) for roundtrip from agent -> device -> agent.

// function that triggers responses if timeout expires
function checkResponses() {
    local newQueue = [];
    // loop through http response queue
    local t = time();
    for (local i = 0; i < responseQueue.len(); i++) {
         // if it's been sitting for more than allowed timeout
         if ((t - r.timestamp) > timeout) {
              // send a timeout error
              r.response.send(408, "Device timed out");
              server.log("Request " + r.timestamp + " timed-out.");
         } else {
             // otherwise, keep it around
             newQueue.push(r);
         }
    }

    // get rid of all the expired responses 
    // (note: this is a really lazy way of doing this)
    responseQueue = newQueue;

    // call this function again in a second
    imp.wakeup(checkResponses, 1.0);
} 

// when we get an HTTP request
http.onrequest(function(req, resp) {
    // create object for queue and push it onto it
    local queueObject = { timestamp = time(), response = resp };
    responseQueue.push(queueObject);

    // send message to see if device is online
    device.send("ping", queueObject.timestamp);
});

// when we get a message saying the device is online
device.on("pong", function(timestamp) {
    // check to see if the originating request is still in the queue
    foreach(r in responseQueue) {
        // if it is, send a 200 response
        if (r.timestamp == timestamp) {
             r.resp.send(200, "OK");
             return;
        }
    }
    server.log("Request " + timestamp + " already timed out");
});

device.nut

// when we get a ping message
agent.on("ping", function(timestamp) {
    // do something
    
    // send a pong
    server.send("pong", timestamp);
}

Nothing is triggering the "checkReponses()" method. Am I missing something? Also, shouldn't the request body be parsed or stored away as well?