Chrome Devpanel Extension Communicating with Background Page

I have an extension to the chrome devtools panel. I can send messages to the page using chrome.devtools.inspectedWindow.eval ... but how do I receive messages in the dev panel? Specifically, I need my devpanel to hook into events that happen on the page. I can't get it to listen to events on my content script, nor the background page.

I've tried chrome.extension.sendMessage in the content script, along with chrome.extension.onMessage.addListener in the dev panel script. But sendMessage complains with Port error: Could not establish connection. Receiving end does not exist.

The issue persists with long-lived connections:

In content script or background page:

var port = chrome.extension.connect({name: "test"});
port.postMessage({msg: "testing"});

In dev tools panel javascript:

chrome.extension.onConnect.addListener(function(port) {
    port.onMessage.addListener(function(msg) {
         // never gets here
    });
 });

How can I listen for events that are triggered in my content script-- in my dev tool panel? A diagram like this from Firefox's Add-On SDK would be great: https://addons.mozilla.org/en-US/developers/docs/sdk/latest/static-files/media/content-scripting-overview.png