How to reply from outside of the hapi.js route handler

I have a hapi.js route where I want to defer the response. I've tried storing the reply function and calling it later, or wrapping it in a promise, but hapi always responds immediately with a 500 Internal Server Error response.

Store the reply for later:

var pendingReplies = {};
server.route({
    method: "POST",
    path: "/",
    handler: function (request, reply) {
        var id = generateId();
        pendingReplies[id] = reply;
    }
});
... // reply later by calling:
function sendResponse(id, data) {
    pendingReplies[id](data);
}

I've tried creating a promise to reply:

handler: function (request, reply) {
    var id = generateId();
    return new Promise(function (resolve, reject) {
        pendingReplies[id] = resolve;
    }).then(function (data) {
        return reply(data);
    });
}

I've tried using reply().hold()

handler: function (request, reply) {
    var id = generateId();
    var response = reply().hold();
    pendingReplies[id] = function (data) {
        response.source = data;
        response.send();
    };
    return response;
}

I've tried using reply().hold() with a Promise:

handler: function (request, reply) {
    var id = generateId();
    var response = reply().hold();
    return new Promise(function (resolve, reject) {
        pendingReplies[id] = resolve;
    }).then(function (data) {
        response.source = data;
        response.send();
        return response;
    });
}

With each of these, as soon as the handler function exits, I get a 500 response.

Is it possible with hapi to reply to a request asynchronously from outside of the route handler?