Identify requests coming from PageWorker

Is it possible, from within the "http-on-modify-request" event, to identify which requests are coming from a PageWorker object, as opposed to those coming from visible tabs/windows?

Note: Because of redirects and subresources, the URL here is NOT the same URL as the pageWorkers contentURL property.

require("sdk/system/events").on("http-on-modify-request", function(e) {

    var httpChannel = e.subject.QueryInterface(Ci.nsIHttpChannel), 
        url = httpChannel.URI.spec, 
        origUrl = httpChannel.originalURI.spec;

    ...         
});